atf: Fix builds of native tools during build process

atf: Fix builds of native tools during build process

atf builds some native tools such as fiptool during its build process,
this should use Yocto's native toolchain options instead of just the
host's.

This way the fiptool build will actually use the openssl-native dependency
from Yocto's native sysroot, instead of the host's openssl (which may not
exist).

Signed-off-by: Daniel Klauer <daniel.klauer@gin.de>