coreboot-sdk: Prefer gnat package over gnat-6

coreboot-sdk: Prefer gnat package over gnat-6

The choice of `gnat-6` was originally an optimization because the meta-
package `gnat` installs not only the current GNAT version but also other
unwanted (and hard to explain) dependencies. Later it was necessary
because GCC 8 couldn't compile our older crossgcc.

Now that we switched crossgcc to GCC 8.1, `gnat` should be fine.

Change-Id: Ica8a1f9d6d71a74ffc4ec76aa0cfbe4b604cde1b
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/29454
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>