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/P1021RDB-PC_NAND_defconfig b/configs/P1021RDB-PC_NAND_defconfig
index 9ad4983..ae9de3d 100644
--- a/configs/P1021RDB-PC_NAND_defconfig
+++ b/configs/P1021RDB-PC_NAND_defconfig
@@ -12,6 +12,7 @@
 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