toolchains: mips: prevent failing build on MacOS

mips gcc will fail to build using the system's
BSD sed. Make sure the Homebrew-installed GNU sed
'sed' simlink is in the PATH. Otherwise, only 'gsed'
can be used to run GNU sed by default.

Change-Id: I0a793c692d9a6de4f326e447281892eb08e4f4cb

tools/rockboxdev.sh

@@ -14,6 +14,7 @@ if [ "$system" == "Darwin" ]; then
     READLINK=greadlink
     TMP="$TMPDIR"
     SED=gsed
+    PATH="$HOMEBREW_PREFIX/opt/gnu-sed/libexec/gnubin:${PATH}"
 else
     READLINK=readlink
     parallel=`nproc`