ramips, mt7620: reproducible elecom-header uid/gid

ramips, mt7620: reproducible elecom-header uid/gid

The elecom-header renames the firmware image to v_0.0.0.bin, stores its
MD5 sum as v_0.0.0.md5 and tars both files again.

Both v_0.0.0 files are created as the build user making it harder to
reproduce.

This commit sets the owner/group of both files to root by adding extra
options to the final tar command.

Before:
0 buildbot   (101) buildbot   (102)  3932164 2019-11-05 14:43:22.000000 v_0.0.0.bin
0 buildbot   (101) buildbot   (102)       33 2019-11-05 14:43:22.000000 v_0.0.0.md5

After:
0 root         (0) root         (0)  3932164 2019-11-05 23:43:08.000000 v_0.0.0.bin
0 root         (0) root         (0)       33 2019-11-05 23:43:08.000000 v_0.0.0.md5

Signed-off-by: Paul Spooren <mail@aparcar.org>