imx8mm-evk: Generate a single bootable flash.bin again

After the conversion to binman in commit 8996e6b7 ("imx8mm_evk: switch
to use binman to pack images"), it is necessary to flash both flash.bin and
u-boot.itb to get a bootable system. Prior to this commit, only flash.bin
was needed.

Such new requirement breaks existing distro mechanisms to generate the
final binary because the extra u-boot.itb is now required.

Generate a final flash.bin that can be used again as a single
bootable binary to keep the original behavior.

After this change the SPL binary is called spl.bin, which is a more
descriptive name for its purpose, and can still be used standalone
(for example, for secure boot purposes).

Also update imx8mm_evk.rst to remove the u-boot.itb copy step.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>