libpayload: Add an exec() and i386_do_exec() function

Add functions for libpayload to execute other payloads in memory,
and have those functions return cleanly.

Signed-off-by: Jordan Crouse <jordan.crouse@amd.com>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>


git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3338 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1

payloads/libpayload/i386/head.S

@@ -63,13 +63,14 @@ _init:
 
 	/* Let's rock. */
 	call start_main
-	
+
+	/* %eax has the return value - pass it on unmolested */
 _leave:
 	/* Get old stack pointer. */
 	popl %ebx
 
 	/* Restore old stack. */
-	movl %esp, %ebx
+	movl %ebx, %esp
 
 	/* Return to the original context. */
-	lret
+	ret