ath79: set lzma-loader variables to null by default

ath79: set lzma-loader variables to null by default

This fixes a small regression where the lzma-loader variable values
are being shared between boards that require different configurations.

If not set to "" globally, a device without these settings will just take
the last values another device has set before in the queue.

Fixes: 1b8bd17c2d07 ("ath79: lzma-loader: allow setting custom kernel magic")
Signed-off-by: Michael Pratt <mcpratt@pm.me>
[add detailed explanation to the commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>