-mXXX option is deprecated already in arc-2016.03 toolchain
and removed completely starting from arc-2016.09.
Direct replacement is -mcpu=XXX which is already supported by
arc-2016.03 used today in Lede.
With that change we'll be ready for ARC toolchain update still
keeping everything working with current tools.
Signed-off-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: John Crispin <john@phrozen.org>