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>

include/asm-generic/pe.h

@@ -0,0 +1,21 @@
+/*
+ *  Portable Executable and Common Object Constants
+ *
+ *  Copyright (c) 2018 Heinrich Schuchardt
+ *
+ *  based on the "Microsoft Portable Executable and Common Object File Format
+ *  Specification", revision 11, 2017-01-23
+ *
+ *  SPDX-License-Identifier:     GPL-2.0+
+ */
+
+#ifndef _ASM_PE_H
+#define _ASM_PE_H
+
+/* Subsystem type */
+#define IMAGE_SUBSYSTEM_EFI_APPLICATION		10
+#define IMAGE_SUBSYSTEM_EFI_BOOT_SERVICE_DRIVER	11
+#define IMAGE_SUBSYSTEM_EFI_RUNTIME_DRIVER	12
+#define IMAGE_SUBSYSTEM_EFI_ROM			13
+
+#endif /* _ASM_PE_H */