Add an option for Waiting for gdb connection if the gdb stub configuration is chosen.

Here's a quick demonstration on how to use it(tested on M4A785T-M).
  (gdb) file ./build/cbfs/fallback/coreboot_ram.debug
  Reading symbols from [...]/build/cbfs/fallback/coreboot_ram.debug...done.
  (gdb) set remotebaud 115200
  (gdb) target remote /dev/ttyUSB0
  Remote debugging using /dev/ttyUSB0
  _text () at src/arch/x86/lib/c_start.S:85
  85		call	hardwaremain

Change-Id: Ia49cbecc41deb061433bc39f5b81715da49edc98
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Reviewed-on: http://review.coreboot.org/1134
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -779,6 +779,13 @@ config GDB_STUB
 	  If enabled, you will be able to set breakpoints for gdb debugging.
 	  See src/arch/x86/lib/c_start.S for details.
 
+config GDB_WAIT
+	bool "Wait for a GDB connection"
+	default n
+	depends on GDB_STUB
+	help
+	  If enabled, coreboot will wait for a GDB connection.
+
 config DEBUG_CBFS
 	bool "Output verbose CBFS debug messages"
 	default n

src/arch/x86/lib/c_start.S

@@ -78,6 +78,9 @@ _start:
 	/* Restore the stack location */
 	movl	%ebp, %esp
 
+#if CONFIG_GDB_WAIT == 1
+	call gdb_stub_breakpoint
+#endif
 	/* The boot_complete flag has already been pushed */
 	call	hardwaremain
 	/* NOTREACHED */
@@ -228,7 +231,7 @@ int_hand:
 
 	iret
 
-#if CONFIG_GDB_STUB == 1
+#if CONFIG_GDB_WAIT == 1
 
 	.globl gdb_stub_breakpoint
 gdb_stub_breakpoint: