imx: Fix automatic make targets for imx images

Automatically build the 'u-boot.imx' (i.e. imx header + u-boot.bin) and 'SPL'
(i.e. imx header + u-boot-spl.bin) make targets for all imx processors
supporting this header, so for arm926ejs, arm1136 and armv7. Some combinations
were missing.

At the same time, fix the build of SPL targets not supporting the imx header on
arm1136. For arm1136, the 'SPL' make target was forced to build in all cases if
CONFIG_SPL_BUILD was defined, even for non-imx platforms or imx setups without
an imx header.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>