assets: Add missing mips/ symlink

Bug #195 added MIPS binaries, but the assets/ symlink was missing.  This
means that update.zip still worked as expected on MIPS, but MIPS binaries
were missing from the apk.
Merge pull request #213 from cernekee/mips-missing-symlink-v1

assets: Add missing mips/ symlink
Merge remote-tracking branch 'upstream/master' into kk-mr0