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/at91sam9g10ek_nandflash_defconfig b/configs/at91sam9g10ek_nandflash_defconfig
index fff8ee9..e9588d2 100644
--- a/configs/at91sam9g10ek_nandflash_defconfig
+++ b/configs/at91sam9g10ek_nandflash_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SKIP_LOWLEVEL_INIT=y
 CONFIG_ARCH_AT91=y
 CONFIG_SYS_TEXT_BASE=0x21f00000
 CONFIG_TARGET_AT91SAM9261EK=y