Makefile: fix dependency for imx targets

Makefile: fix dependency for imx targets

imx targets are defined in arch/arm/mach-imx/Makefile.
Some of them are dependent on targets defined in main
Makefile. For the Makefile in arch/arm/mach-imx this
targets must be finished before the imx targets are
build, if not you get for example the error:

make -f /home/hs/abb/mainlining/u-boot/scripts/Makefile.build obj=arch/arm/mach-imx u-boot-dtb.imx
make[2]: *** No rule to make target 'u-boot-fit-dtb.bin', needed by 'u-boot-dtb.imx'.  Stop.
make[1]: *** [/home/hs/abb/mainlining/u-boot/Makefile:1123: u-boot-dtb.imx] Error 2
make[1]: *** Waiting for unfinished jobs....
make[1]: Leaving directory '/work/hs/compile/u-boot/aristainetos2_defconfig'
make: *** [Makefile:148: sub-make] Error 2
compile failed

In above case of CONFIG_MULTI_DTB_FIT is defined, the
u-boot-dtb.imx is dependent on the u-boot-fit-dtb.bin
which may is not build yet ...

I could reproduce this error on a travis build also if
I build an out-of-tree build on a local machine with a
build directory on a "slow" slow storage device. If
building the same source target with a build dir on a
fast storage device, the build works.

I found no solution to tell the arch/arm/mach-imx/Makefile
to find the targets in main Makefile, if there is a way
this would be the better fix.

I solved it by adding a IMX_DEPS var, which holds a list
of main u-boot targets, which must be finished, before
calling imx targets and fixed the build for imx
targets which enabled CONFIG_MULTI_DTB_FIT.

I think it is just luck, that imx targets with
CONFIG_OF_SEPARATE enabled build, because the
u-boot-dtb.imx target depends on u-boot-dtb.bin
which gets build early enough before starting with
u-boot-dtb.imx. May this targets should be fixed too.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>