opkg: switch to own fork to improve memory usage

Switch to our own fork of opkg to significantly reduce the required amount
of memory when updating lists or installing packages.

Preliminary tests showed a usage drop of about 90% during these operations,
from ~3.7MB with unmodified opkg to ~360KB with our custom fork.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>