Makefile.inc: Generate master header and pointer as C structs

The makefiles don't like cbfs file names with spaces in them so update
the file name with '_' instead of spaces. To keep the master header at
the top of cbfs, add a placeholder.

This removes the need to handle the cbfs master header in cbfstool.
This functionality will be dropped in a later CL.

On x86 reserve some space in the linker script to add the pointer.
On non-x86 generate a pointer inside a C struct file.

As a bonus this would actually fix the master header pointer mechanism
on Intel/APL as only the bootblock inside IFWI gets memory mapped.

TESTED on thinkpad X201: SeaBIOS correctly finds the cbfs master
header.

Change-Id: I3ba01be7da1f09a8cac287751497c18cda97d293
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59132
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/arch/x86/bootblock.ld

@@ -73,9 +73,14 @@ SECTIONS {
 	_X86_RESET_VECTOR = .;
 	.reset . : {
 		*(.reset);
-		. = 15;
-		BYTE(0x00);
+		. = _X86_RESET_VECTOR_FILLING;
+		BYTE(0);
 	}
+	. = 0xfffffffc;
+	.header_pointer . : {
+		KEEP(*(.header_pointer));
+	}
+	_X86_RESET_VECTOR_FILLING = 15 - SIZEOF(.header_pointer);
 	_ebootblock = .;
 }