gnu-efi: update to ab54e2b4

This allows gnu-efi to be built outside the source tree properly.

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
efi: fix up gnu-efi build

This eliminates the forking and pushes the build under the efi32/ and
 efi64/ object directories eliminating a build race and preventing the
 build from breaking as gnu-efi takes a few seconds to complete.

Depends on gnu-efi commit 52d88dd

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
mk/efi.mk: Add libefi.a dependency

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>