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/bcm7260_defconfig b/configs/bcm7260_defconfig
index a89014b..62b2d27 100644
--- a/configs/bcm7260_defconfig
+++ b/configs/bcm7260_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SKIP_LOWLEVEL_INIT=y
 CONFIG_ARCH_BCMSTB=y
 CONFIG_SYS_TEXT_BASE=0x10100000
 CONFIG_TARGET_BCM7260=y