libpayload: Save EAX and EBX only for multiboot payloads

When CONFIG_LP_MULTIBOOT is enabled, save the values of EAX and EBX
passed from the bootloader. This information can be useful for
multiboot payloads feature alone.

Change-Id: I98c2cd00206ee48eb0fc67edd9533032bcf3e5eb
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82040
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Reviewed-by: Dinesh Gehlot <digehlot@google.com>

payloads/libpayload/arch/x86/head.S

@@ -63,9 +63,11 @@ _init:
 	/* No interrupts, please. */
 	cli
 
+#if CONFIG(LP_MULTIBOOT)
 	/* Store EAX and EBX */
 	movl %eax, loader_eax
 	movl %ebx, loader_ebx
+#endif
 
 	/* save pointer to coreboot tables */
 	movl 4(%esp), %eax