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/m53evk_defconfig b/configs/m53evk_defconfig
index 653b330..0b5e01d 100644
--- a/configs/m53evk_defconfig
+++ b/configs/m53evk_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_M53EVK=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/denx/m53evk/imximage.cfg"
 CONFIG_BOOTDELAY=3