buildroot

toolchain-external: do not use BR2_GCC_TARGET_TUNE anymore

Since the BR2_GCC_TARGET_TUNE value is always empty now, there is no
longer a point in using it in the external toolchain logic.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>