[efi] Fix .efi and .efidrv linking

[efi] Fix .efi and .efidrv linking

The linker chooses to look for _start first and always picks
efidrvprefix.o to satisfy it (probably because it's earlier in the
archive) which causes a multiple definition error when the linker
later has to pick efiprefix.o for other symbols.

Fix by using EFI-specific TGT_LD_FLAGS with an explicit entry point.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>