Force prereq again in SDK in order to fix GCC and Python dangling
symlinks:
staging_dir/host/bin/g++ -> /builder/ath79_generic/ccache_cxx.sh
staging_dir/host/bin/gcc -> /builder/ath79_generic/ccache_cc.sh
staging_dir/host/bin/python -> /usr/bin/python3.5
staging_dir/host/bin/python3 -> /usr/bin/python3.5
Ref: FS#2424
Signed-off-by: Petr Štetiar <ynezz@true.cz>