syscnf: fix root_dir path to be absolute

We do change directory and it makes sense to use absolute path either
way so let's go for it.
6 jobs for fix-ipkg-root in 3 minutes and 11 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #200208
cppcheck

00:00:28

passed #200209
luacheck

00:00:30

passed #200206
system-tests

00:01:35

passed #200207
system-valgrind-tests

00:03:11

passed #200204
unit-tests

00:01:17

passed #200205
unit-valgrind-tests

00:02:36