docs/manual: update to mention eglibc support in Buildroot toolchain backend

by itself a cross-compilation toolchain, before building the userspace

applications and libraries for your target embedded system.

This backend is the historical backend of Buildroot, and is limited to

the usage of the http://www.uclibc.org[uClibc C library] (i.e, the

_glibc_ and _eglibc_ C libraries are not supported by this backend,

see the _External toolchain backend_ and _Crosstool-NG toolchain

backend_ for solutions to use either _glibc_ or _eglibc_).

This backend is the historical backend of Buildroot, and has been

limited for a long time to the usage of the

http://www.uclibc.org[uClibc C library]. Support for the _eglibc_ C

library has been added in 2013 and is at this point considered

experimental. See the _External toolchain backend_ and _Crosstool-NG

toolchain backend_ for other solutions to use _glibc_ or _eglibc_.

Once you have selected this backend, a number of options appear. The

most important ones allow to:

   the C library might be using interfaces that are not provided by

   your Linux kernel.

 * Change the version and the configuration of the uClibc C

   library. The default options are usually fine. However, if you

   really need to specifically customize the configuration of your

   uClibc C library, you can pass a specific configuration file

   here. Or alternatively, you can run the +make uclibc-menuconfig+

   command to get access to uClibc's configuration interface. Note

   that all packages in Buildroot are tested against the default

   uClibc configuration bundled in Buildroot: if you deviate from this

   configuration by removing features from uClibc, some packages may

   no longer build.

 * Change the version and the configuration of the uClibc C library

   (if uClibc is selected). The default options are usually

   fine. However, if you really need to specifically customize the

   configuration of your uClibc C library, you can pass a specific

   configuration file here. Or alternatively, you can run the +make

   uclibc-menuconfig+ command to get access to uClibc's configuration

   interface. Note that all packages in Buildroot are tested against

   the default uClibc configuration bundled in Buildroot: if you

   deviate from this configuration by removing features from uClibc,

   some packages may no longer build.

 * Change the version of the GCC compiler and binutils.

 * Select a number of toolchain options: whether the toolchain should

   have largefile support (i.e support for files larger than 2 GB on

   32 bits systems), IPv6 support, RPC support (used mainly for NFS),

   wide-char support, locale support (for internationalization), C++

   support, thread support. Depending on which options you choose, the

   number of userspace applications and libraries visible in Buildroot

   menus will change: many applications and libraries require certain

   toolchain options to be enabled. Most packages show a comment when

   a certain toolchain option is required to be able to enable those

   packages.

 * Select a number of toolchain options (uClibc only): whether the

   toolchain should have largefile support (i.e support for files

   larger than 2 GB on 32 bits systems), IPv6 support, RPC support

   (used mainly for NFS), wide-char support, locale support (for

   internationalization), C++ support, thread support. Depending on

   which options you choose, the number of userspace applications and

   libraries visible in Buildroot menus will change: many applications

   and libraries require certain toolchain options to be enabled. Most

   packages show a comment when a certain toolchain option is required

   to be able to enable those packages.

It is worth noting that whenever one of those options is modified,

then the entire toolchain and system must be rebuilt. See

* Rebuilding the toolchain is needed when doing +make clean+, which

  takes time. If you're trying to reduce your build time, consider

  using the _External toolchain backend_.

* Limited to the _uClibc_ C library.

[[external-toolchain-backend]]

External toolchain backend