Makefile: Build final mpc85xx non-SPL image in standard file u-boot.bin

Currently Makefile produces final mpc85xx image when SPL is not used in
custom file u-boot-with-dtb.bin. It is quite confusing name as build
process produce also intermediate file standard file u-boot-dtb.bin (which
is just intermediate and not bootable). Other platforms use u-boot.bin
(UBOOT_BIN) as standard name for final bootable raw image.

So change Makefile rules and binman to produce final bootable file for
mpc85xx also into file u-boot.bin. There is just need for mpc85xx to not
define default rule for u-boot.bin then instruct binman (via DTS file) to
store final image into u-boot.bin (instead of u-boot-with-dtb.bin) and
finally rename target u-boot-with-dtb.bin to u-boot.bin.

With this change are also removed custom Makefile hacks for mpc85xx that it
produced non-standard output file. And also updated documentation.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Marek Behún <kabel@kernel.org>