u-boot: Fix building for ARC700

u-boot: Fix building for ARC700

With newer ARC toolchain obsolete -marcXXX were finally deperecated
and so compiler throws errors now about unknown option.

Solution is as simple as switching to more generic -mcu-XXX
options. Which we do. Unfortunately that change in upstream U-Boot [1]
happened right after v2016.09 was released an so we need to have that fix
for v2016.07 which is mentioned in axs10x defconfigs and for the latest
U-Boot release v2016.09 (which is selected by default if no defconfig is
used).

Once we deprecate either U-Boot version in U-Boot corresponding patch
should be removed essentially.

[1] http://git.denx.de/?p=u-boot.git;a=commit;h=7c8d81605302e7d7fdd3e7d8eb69302bddc64a2c

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: handle the U-Boot bump to 2016.09.01.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>