toolchain-external: fix definition of TOOLCHAIN_EXTERNAL_READELF

toolchain-external: fix definition of TOOLCHAIN_EXTERNAL_READELF

TOOLCHAIN_EXTERNAL_READELF is defined to
$(TOOLCHAIN_EXTERNAL_CROSS)readelf$(TOOLCHAIN_EXTERNAL_SUFFIX), where
TOOLCHAIN_EXTERNAL_SUFFIX is .br_real for Buildroot
toolchains. However, this is bogus, because readelf is not wrapped by
the Buildroot toolchain wrapper, so "<arch>-readelf.br_real" never
exists.

Therefore, it should simply be defined as
$(TOOLCHAIN_EXTERNAL_CROSS)readelf. Currently,
TOOLCHAIN_EXTERNAL_READELF is not used anywhere, so it wasn't visible,
but a follow-up commit will make use of it.

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