efi: Makefile: Export variables for use with EFI

When building an EFI app we need three things:

   - start-up code
   - relocation code
   - link script

These are all different for each architecture. We also need special
compiler flags in some cases.

Add top-level Makefile variables for these along with documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

Makefile

@@ -527,6 +527,15 @@ endif
 endif
 endif
 
+# These are set by the arch-specific config.mk. Make sure they are exported
+# so they can be used when building an EFI application.
+export EFI_LDS		# Filename of EFI link script in arch/$(ARCH)/lib
+export EFI_CRT0		# Filename of EFI CRT0 in arch/$(ARCH)/lib
+export EFI_RELOC	# Filename of EFU relocation code in arch/$(ARCH)/lib
+export CFLAGS_EFI	# Compiler flags to add when building EFI app
+export CFLAGS_NON_EFI	# Compiler flags to remove when building EFI app
+export EFI_TARGET	# binutils target if EFI is natively supported
+
 # If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
 # that (or fail if absent).  Otherwise, search for a linker script in a
 # standard location.