buildroot

toolchain-external: unify LIB_EXTERNAL_LIBS and USR_LIB_EXTERNAL_LIBS

With the alignment of toolchain library location in target and staging,
there is no need anymore for the distinction between LIB_EXTERNAL_LIBS and
USR_LIB_EXTERNAL_LIBS. Unify them into TOOLCHAIN_EXTERNAL_LIBS.

Related, update the help text of
BR2_TOOLCHAIN_EXTRA_TOOLCHAIN_EXTERNAL_LIBS.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>