Fix RISCV64 not setting the subsystem according to EFI_SUBSYSTEM

The RISCV64 toolchain does not support 16-bit variable relocation from assembly,
and even if it did support relocations, it would not properly set the subsystem
from --defsym. So we add an extra step on RISCV64, post objcopy, to set the field
manually (using dd and /bin/echo to output the relevant byte, as GNU Make's echo
does not support -ne).
Closes ncroxon/gnu-efi#27.

Signed-off-by: Pete Batard <pete@akeo.ie>

Make.rules

@@ -36,12 +36,17 @@
 
 .SECONDARY:
 
+CHAR_SUBSYSTEM=$(subst 0x,\x,$(SUBSYSTEM))
+
 ifeq ($(IS_MINGW32),)
 %.efi: %.so
 	@$(ECHO) "  OBJCOPY  $(notdir $@)"
 	$(HIDE)$(OBJCOPY) -j .text -j .sdata -j .data -j .dynamic -j .rodata -j .rel \
 		    -j .rela -j .rel.* -j .rela.* -j .rel* -j .rela* \
 		    -j .areloc -j .reloc $(FORMAT) $*.so $@
+ifeq ($(ARCH),riscv64)
+	$(HIDE)/bin/echo -ne "$(CHAR_SUBSYSTEM)" | dd of=$@ bs=1 seek=156 count=1 conv=notrunc status=none
+endif
 
 %.efi.debug: %.so
 	@$(ECHO) "  OBJCOPY  $(notdir $@)"

gnuefi/crt0-efi-riscv64.S

@@ -16,10 +16,6 @@
  * either version 2 of the License, or (at your option) any later version.
  */
 
-#ifndef EFI_SUBSYSTEM
-#define EFI_SUBSYSTEM 10
-#endif
-
 	.section	.text.head
 
 	/*
@@ -71,7 +67,7 @@ extra_header_fields:
 	// Everything before the kernel image is considered part of the header
 	.4byte	_text - ImageBase		// SizeOfHeaders
 	.4byte	0				// CheckSum
-	.2byte	EFI_SUBSYSTEM			// Subsystem
+	.2byte	0				// Subsystem
 	.2byte	0				// DllCharacteristics
 	.8byte	0				// SizeOfStackReserve
 	.8byte	0				// SizeOfStackCommit