package/rauc: prevent occurring the error when directory exists

Add -p argument that ignore that specified directory already exists.

Fixes:
 mkdir: cannot create directory ‘/home/bartekk/buildroot-2020.11-rc1/output/target/usr/lib/systemd/system/rauc.service.d’: File exists

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>