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/pxm2_defconfig b/configs/pxm2_defconfig
index 2da6bfc..ad7908f 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_PXM2=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y