Convert CONFIG_SPL to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL
Signed-off-by: NTom Rini <trini@konsulko.com>

configs/mx6memcal_defconfig

@@ -14,7 +14,6 @@ CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_SUPPORT_RAW_INITRD=y
-CONFIG_SYS_EXTRA_OPTIONS="SPL"
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y

configs/riotboard_defconfig

@@ -21,7 +21,7 @@ CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="SPL,DDR_MB=1024"
+CONFIG_SYS_EXTRA_OPTIONS="DDR_MB=1024"
 CONFIG_BOOTCOMMAND="run finduuid; run distro_bootcmd"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y