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/lx2160ardb_tfa_defconfig b/configs/lx2160ardb_tfa_defconfig
index f52b26d..5a5685e 100644
--- a/configs/lx2160ardb_tfa_defconfig
+++ b/configs/lx2160ardb_tfa_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SKIP_LOWLEVEL_INIT=y
 CONFIG_GIC_V3_ITS=y
 CONFIG_TARGET_LX2160ARDB=y
 CONFIG_TFABOOT=y