`channel.exec()` doesn't source the shell profile, so PATH may not
include /usr/bin on some routers. Using /usr/bin/opkg explicitly
avoids exit-127 surprises. Added opkg_check() to give a clear error
("firmware may not support package management") before attempting
opkg_update, rather than a confusing "command not found" exit code.
Also split the BusyBox-hostile `grep -v 'all\|noarch'` into two
separate greps for the arch-detection fallback.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>