Make mkdirAndChown do what it claims to do

Currently, mkdirAndChown takes parameters for mode, uid and gid, but
ignores them and hardcodes 0750, AID_SYSTEM, AID_SYSTEM instead.
This doesn't matter much because so far its only invocation passes
exactly those values as parameters -- but clang complains about unused
parameters.

Might as well make the function do what it claims to do.

Change-Id: I5a8056be3278a0c5d0a576dbc061288aa0956a35
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>