ath79: harmonize line breaks in image Makefiles

This harmonizes the line wrapping in image Makefile device
definitions, as those are frequently copy-pasted and are a common
subject of review comments. Having the treatment unifying should
reduce the cases where adjustment is necessary afterwards.

Harmonization is achieved by consistently (read "strictly")
applying certain rules:
- Never put more than 80 characters into one line
- Fill lines up (do not break after 40 chars because of ...)
- Use one tab for indent after wrapping by "\"
- Only break after pipe "|" for IMAGE variables

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>