tools: moveconfig: add --spl option to move options for SPL build

Prior to this commit, the tool could not move options guarded by
CONFIG_SPL_BUILD ifdef conditionals because they do not show up in
include/autoconf.mk.  This new option, if given, makes the tool
parse spl/include/autoconf.mk instead of include/autoconf.mk,
which is probably preferred behavior when moving options for SPL.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>