[build] Fix ARM32 EFI builds with current EDK2 headers

EDK2 commit 6440385 ("MdePkg/Include: Add enumeration size checks to
Base.h") enforced the UEFI specification mandate that enums should
always be 32 bits.  This revealed a latent bug in iPXE, which does not
build with -fno-short-enums.

Fix by adding -fno-short-enums to CFLAGS for ARM32 EFI builds.

Reported-by: Benjamin S. Allen <bsallen@alcf.anl.gov>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/arm32/Makefile.efi

 # -*- makefile -*- : Force emacs to use Makefile mode
 
+# UEFI requires that enums are always 32 bits
+#
+CFLAGS		+= -fno-short-enums
+
 # Specify EFI image builder
 #
 ELF2EFI		= $(ELF2EFI32)