Kconfig: Unify power-after-failure options

The newest and most useful incarnation was hiding in soc/intel/common/.
We move it into the Mainboard menu and extend it with various flags to
be selected to control the default and which options are visible. Also
add a new `int` config MAINBOARD_POWER_FAILURE_STATE that moves the
boolean to int conversion into Kconfig:
  0 - S5
  1 - S0
  2 - previous state

This patch focuses on the Kconfig code. The C code could be unified as
well, e.g. starting with a common enum and safe wrapper around the
get_option() call.

TEST=Did what-jenkins-does with and without this commit and compared
     binaries. Nothing changed for the default configurations.

Change-Id: I61259f864c8a8cfc7099cc2699059f972fa056c0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/29680
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/southbridge/intel/ibexpeak/smihandler.c

@@ -403,7 +403,7 @@ static void southbridge_smi_sleep(void)
 	u8 reg8;
 	u32 reg32;
 	u8 slp_typ;
-	u8 s5pwr = CONFIG_MAINBOARD_POWER_ON_AFTER_POWER_FAIL;
+	u8 s5pwr = CONFIG_MAINBOARD_POWER_FAILURE_STATE;
 
 	// save and recover RTC port values
 	u8 tmp70, tmp72;