Convert CONFIG_SKIP_LOWLEVEL_INIT et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SKIP_LOWLEVEL_INIT
   CONFIG_SKIP_LOWLEVEL_INIT_ONLY

In order to do this, we need to introduce SPL and TPL variants of these
options so that we can clearly disable these options only in SPL in some
cases, and both instances in other cases.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/pg_wcom_expu1_defconfig b/configs/pg_wcom_expu1_defconfig
index f3af8af..ac3165d 100644
--- a/configs/pg_wcom_expu1_defconfig
+++ b/configs/pg_wcom_expu1_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SKIP_LOWLEVEL_INIT=y
 CONFIG_TARGET_PG_WCOM_EXPU1=y
 CONFIG_SYS_TEXT_BASE=0x60100000
 CONFIG_NR_DRAM_BANKS=1