efi_loader: fix building crt0 on arm

Before the patch an undefined constant EFI_SUBSYSTEM was used in the
crt0 code. The current version of binutils does not swallow the error.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=888403

The necessary constant IMAGE_SUBSYSTEM_EFI_APPLICATION is already
defined in pe.h. So let's factor out asm-generic/pe.h for the
image subsystem constants and use it in our assembler code.

IMAGE_SUBSYSTEM_SAL_RUNTIME_DRIVER does not exist in the specification
let's use IMAGE_SUBSYSTEM_EFI_ROM instead.

The include pe.h is only used in code maintained by Alex so let him be the
maintainer here too.

Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/MAINTAINERS b/MAINTAINERS
index be941c1..ac21ec5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -291,6 +291,8 @@
 T:	git git://github.com/agraf/u-boot.git
 F:	doc/README.iscsi
 F:	include/efi*
+F:	include/pe.h
+F:	include/asm-generic/pe.h
 F:	lib/efi*/
 F:	test/py/tests/test_efi*
 F:	cmd/bootefi.c