ramips: mt7620: do not pad sysupgrade Archer images

ramips: mt7620: do not pad sysupgrade Archer images

The current makefile unnecessarily pads sysupgrade image for Archer devices.

This has three implications:
1. higher risk of OOM when uploading the binary image to the device
2. much slower upgrade due to time wasted erasing and writing padding
3. grows image beyond available flash size if metadata are appended

This is already fixed in master, albeit in a completely different way (the
whole target have been reworked)

Fixes: FS#1025, FS#1039

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>