linux-nokia-n900: upgrade to kernel 5.7.19 + build zram module

As discussed with @MartijnBraam in "Pine64 Community Meeting" chat yesterday, the sole purpose of this merge request is to make magnet-less N900 to boot (#1060 (closed)).

The patch was backported to a stable release in 5.7 so it make sense to upgrade the latest release before EOL. Maybe #1135 (closed) can come later since I was not able to get 5.12 booting on my N900 (the display goes blank with backlight and the keyboard leds turns on as soon that happened).

While at it, I also enabled CONFIG_ZRAM so that I could get rid of swapfile on my setup and rely on compressed memory instead.