Convert CONFIG_SPL_LIBGENERIC_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx28evk_auart_console_defconfig b/configs/mx28evk_auart_console_defconfig
index d201619..94df592 100644
--- a/configs/mx28evk_auart_console_defconfig
+++ b/configs/mx28evk_auart_console_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_MX28EVK=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="MXS_AUART,MXS_AUART_BASE=MXS_UARTAPP3_BASE,ENV_IS_IN_MMC"
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y