SPL: Makefile: Build a separate autoconf.mk for SPL

SPL defines CONFIG_SPL_BUILD but this does not percolate to the
autoconf.mk Makefile.  As a result the build breaks when
CONFIG_SPL_BUILD is used in the board-specific include header file. With
this, there is a possibility of having a CONFIG option defined in the
header file but not defined in the Makefile causing all kinds of build
failure and problems.

It also messes things for up, for example, when one might want to
undefine options to keep the SPL small and doesn't want to be stuck with
the CONFIG options used for U-boot.  Lastly, this also avoids defining
special CONFIG_SPL_ variables for cases where some options are required
in U-boot but not in SPL.

We add a spl-autoconf.mk rule that is generated for SPL with the
CONFIG_SPL_BUILD flag and conditionally include it for SPL builds.

Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Ying Zhang <b40530@freescale.com>