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/T2080RDB_NAND_defconfig b/configs/T2080RDB_NAND_defconfig
index e3a149e..a573baf 100644
--- a/configs/T2080RDB_NAND_defconfig
+++ b/configs/T2080RDB_NAND_defconfig
@@ -1,5 +1,6 @@
 CONFIG_PPC=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_MPC85xx=y