post: Migrate to Kconfig

We move the existing CONFIG_POST_* functionality over to CFG_POST and
then introduce CONFIG_POST to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/P2041RDB_NAND_defconfig b/configs/P2041RDB_NAND_defconfig
index 3d24b1a..f495205 100644
--- a/configs/P2041RDB_NAND_defconfig
+++ b/configs/P2041RDB_NAND_defconfig
@@ -112,3 +112,4 @@
 CONFIG_USB_STORAGE=y
 CONFIG_ADDR_MAP=y
 CONFIG_SYS_NUM_ADDR_MAP=64
+CONFIG_POST=y