external toolchain: more documentation about the principles

#

# This file implements the support for external toolchains, i.e

# toolchains that have not been produced by Buildroot itself and that

# are already available on the system on which Buildroot runs.

#

# The basic principle is the following

#

#  1. Perform some checks on the conformity between the toolchain

#  configuration described in the Buildroot menuconfig system, and the

#  real configuration of the external toolchain. This is for example

#  important to make sure that the Buildroot configuration system

#  knows whether the toolchain supports RPC, IPv6, locales, large

#  files, etc. Unfortunately, these things cannot be detected

#  automatically, since the value of these options (such as

#  BR2_INET_RPC) are needed at configuration time because these

#  options are used as dependencies for other options. And at

#  configuration time, we are not able to retrieve the external

#  toolchain configuration.

#

#  2. Copy the libraries needed at runtime to the target directory,

#  $(TARGET_DIR). Obviously, things such as the C library, the dynamic

#  loader and a few other utility libraries are needed if dynamic

#  applications are to be executed on the target system.

#

#  3. Copy the libraries and headers to the staging directory. This

#  will allow all further calls to gcc to be made using --sysroot

#  $(STAGING_DIR), which greatly simplifies the compilation of the

#  packages when using external toolchains. So in the end, only the

#  cross-compiler binaries remains external, all libraries and headers

#  are imported into the Buildroot tree.

#

# Copy a toolchain library and its symbolic links from the sysroot

# directory to the target directory. Also optionaly strips the