toolchain-external: improve help text of some options

	# For the time being, we assume that all custom external

	# toolchains have shadow password support.

	select BR2_TOOLCHAIN_HAS_SHADOW_PASSWORDS

	help

	  Select this option if your external toolchain uses the

	  uClibc C library (available from http://www.uclibc.org/).

config BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC

	bool "glibc/eglibc"

	select BR2_TOOLCHAIN_EXTERNAL_GLIBC

	help

	  Note: eglibc is a variant of glibc that (among other things) can be

	  configured to exclude some of its features. Using a toolchain with

	  eglibc configured to exclude key features may cause build failures to

	  some packages.

	  Select this option if your external toolchain uses the GNU C

	  library (available from https://www.gnu.org/software/libc/)

	  or its variant the eglibc library (http://www.eglibc.org/).

	  Note: eglibc is a variant of glibc that (among other things)

	  can be configured to exclude some of its features. Using a

	  toolchain with eglibc configured to exclude key features may

	  cause build failures to some packages.

config BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL

	bool "musl"