toolchain-external: update comments

#

#  * Toolchains generated by Crosstool-NG

#  * Toolchains generated by Buildroot

#  * ARM, MIPS and PowerPC toolchains made available by

#    Codesourcery. For the MIPS toolchain, the -muclibc variant isn't

#    supported yet, only the default glibc-based variant is.

#  * Toolchains provided by Linaro for the ARM and AArch64

#    architectures

#  * Sourcery CodeBench toolchains (from Mentor Graphics) for the ARM,

#    MIPS, PowerPC, x86, x86_64 and NIOS 2 architectures. For the MIPS

#    toolchain, the -muclibc variant isn't supported yet, only the

#    default glibc-based variant is.

#  * Analog Devices toolchains for the Blackfin architecture

#  * Xilinx toolchains for the Microblaze architecture

#

# The basic principle is the following

#

#  1. If the toolchain is not pre-installed, download and extract it

#  in $(TOOLCHAIN_EXTERNAL_INSTALL_DIR).

#  in $(TOOLCHAIN_EXTERNAL_INSTALL_DIR). Otherwise,

#  $(TOOLCHAIN_EXTERNAL_INSTALL_DIR) points to were the toolchain has

#  already been installed by the user.

#

#  2. For all external toolchains, perform some checks on the

#  conformity between the toolchain configuration described in the

# Details about sysroot directory selection.

#

# To find the sysroot directory:

# To find the sysroot directory, we use the trick of looking for the

# 'libc.a' file with the -print-file-name gcc option, and then

# mangling the path to find the base directory of the sysroot.

#

#  * We first try the -print-sysroot option, available in gcc 4.4.x

#    and in some Codesourcery toolchains.

# Note that we do not use the -print-sysroot option, because it is

# only available since gcc 4.4.x, and we still support 4.2.x (for

# AVR32) and 4.3.x.

#

#  * If this option is not available, we fallback to the value of

#    --with-sysroot as visible in CROSS-gcc -v.

#

# When doing those tests, we don't pass any option to gcc that could

# select a multilib variant (such as -march) as we want the "main"

# sysroot, which contains all variants of the C library in the case of

# multilib toolchains. We use the TARGET_CC_NO_SYSROOT variable, which

# is the path of the cross-compiler, without the

# --sysroot=$(STAGING_DIR), since what we want to find is the location

# of the original toolchain sysroot. This "main" sysroot directory is

# stored in SYSROOT_DIR.

# When doing this, we don't pass any option to gcc that could select a

# multilib variant (such as -march) as we want the "main" sysroot,

# which contains all variants of the C library in the case of multilib

# toolchains. We use the TARGET_CC_NO_SYSROOT variable, which is the

# path of the cross-compiler, without the --sysroot=$(STAGING_DIR),

# since what we want to find is the location of the original toolchain

# sysroot. This "main" sysroot directory is stored in SYSROOT_DIR.

#

# Then, multilib toolchains are a little bit more complicated, since

# they in fact have multiple sysroots, one for each variant supported