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/P1020RDB-PC_36BIT_NAND_defconfig b/configs/P1020RDB-PC_36BIT_NAND_defconfig
index b76b489..642bea1 100644
--- a/configs/P1020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P1020RDB-PC_36BIT_NAND_defconfig
@@ -13,6 +13,7 @@
 CONFIG_TPL_ENV_SUPPORT=y
 CONFIG_TPL_I2C_SUPPORT=y
 CONFIG_TPL_LIBCOMMON_SUPPORT=y
+CONFIG_TPL_LIBGENERIC_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y