[build] Allow binaries to request a log partition from genfsimg

For UEFI, the USB disk image is constructed from the built EFI binary
(e.g. bin-x86_64-efi/ipxe.efi) by genfsimg, which does not itself have
any way to access the build configuration.  We therefore need a way to
annotate the binary such that genfsimg can determine whether or not to
include a log partition within the USB disk image.

The "OEM ID" and "OEM information" fields within the PE header can be
used for this, since they are easily accessed and serve no other
purpose.  We define bit 0 of "OEM information" as a flag indicating
that a log partition should be included.  If this bit is set, genfsimg
will create a log partition with a layout matching that of the BIOS
build (i.e. using partition 3 and at an offset of 16kB from the start
of the disk).

The PE header is constructed by elf2efi.c, which takes as an input the
linked ELF form of the binary.  We use an ELF .note section to allow
any linked-in object to communicate the log partition request through
to elf2efi.c, which then populates the OEM information field
accordingly.

We choose to use the same field locations within the BIOS bzImage
header, since this allows genfsimg to use the same logic for both BIOS
and UEFI binaries.  In a BIOS build, there is no external processing
equivalent to elf2efi.c, and so we construct the field value directly
using absolute symbols and explicit relocation records.

(Note that the bzImage header is relevant only when using genfsimg to
construct a combined BIOS/UEFI image.  In the common case of building
a BIOS-only image such as bin/ipxe.usb, the partition table is
manually constructed by usbdisk.S and genfsimg is not involved.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/compiler.h

@@ -166,6 +166,31 @@
 
 /** @} */
 
+/**
+ * @defgroup ipxenote Macros to provide ELF notes
+ * @{
+ */
+
+/* Construct an iPXE-specific ELF note */
+#define IPXE_NOTE( type )					\
+	__asm__ ( ".section \".note.ipxe\", \"\", "		\
+		  _S2 ( ASM_TCHAR ) "note\n\t"			\
+		  /* Owner name length */			\
+		  ".long 4\n\t"					\
+		  /* Content length */				\
+		  ".long 0\n\t"					\
+		  /* Type */					\
+		  ".long " _S2 ( _C2 ( IPXE_NOTE_, type ) )	\
+		  "\n\t"					\
+		  /* Owner name */				\
+		  ".ascii \"iPXE\"\n\t"				\
+		  ".previous\n\t" )
+
+/** Build will use a disk-based console log, if present */
+#define IPXE_NOTE_DISKLOG 0x18aed109
+
+/** @} */
+
 /**
  * @defgroup objmacros Macros to provide or require explicit objects
  * @{