ovmf: unify DEPENDS

Instead of depending on iasl-native, depend on ovmf-native as iasl was merged
into that recipe some time ago.

bc-native doesn't appear to be a build requirement anymore, and for clarity
merge two overridden DEPENDS into a single DEPENDS.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>