x86: Emit a post code before booting

It is possible to benchmark U-Boot using QEMU as described here:

   https://github.com/stefano-garzarella/qemu-boot-time

On x86 devices, that script can read post codes from a perf trace. Add
a code which is emitted just before handing off to the Linux kernel.
Existing post codes can be used to determine when U-Boot starts.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/x86/include/asm/post.h

@@ -31,6 +31,7 @@
 #define POST_DRAM		0x30
 #define POST_LAPIC		0x31
 #define POST_OS_RESUME		0x40
+#define POST_BOOT		0x42
 
 #define POST_RAM_FAILURE	0xea
 #define POST_BIST_FAILURE	0xeb

arch/x86/lib/bootm.c

@@ -24,6 +24,7 @@
 #include <asm/bootparam.h>
 #include <asm/cpu.h>
 #include <asm/byteorder.h>
+#include <asm/post.h>
 #include <asm/zimage.h>
 #ifdef CONFIG_SYS_COREBOOT
 #include <asm/arch/timestamp.h>
@@ -201,6 +202,8 @@ int boot_linux_kernel(ulong setup_base, ulong entry, bool image_64bit)
 	if (IS_ENABLED(CONFIG_EFI_APP))
 		return efi_boot(setup_base, entry, image_64bit);
 
+	post_code(POST_BOOT);
+
 	if (image_64bit) {
 		if (!cpu_has_64bit()) {
 			puts("Cannot boot 64-bit kernel on 32-bit machine\n");