toolchain-external: fix user provided libraries deployment

In commit 919b4f9e the internal symbol
LIB_EXTERNAL_LIBS was renamed TOOLCHAIN_EXTERNAL_LIBS but the find and
replace command also renamed BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS to
BR2_TOOLCHAIN_EXTRA_TOOLCHAIN_EXTERNAL_LIBS which doesn't exist.

So user provided libraries defined in BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS
are not copied anymore to staging and target directories.

For example:
BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS="libasan.* libubsan.*"

Simply revert this change by renaming
BR2_TOOLCHAIN_EXTRA_TOOLCHAIN_EXTERNAL_LIBS to BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>