opkg: switch to LEDE fork (#120, #551, #571)

Cherry-pick the following commits from master to bump opkg in LEDE 17.01:

b65dc04712 opkg: switch to own fork to improve memory usage
55ffc38004 opkg: re-enable usign support
19720a6f03 opkg: fix handling conffiles in status lists
9e4555f58d opkg: fix stray printf() (#551)
ebf846b005 opkg: mark as essential (FS#571)
aedd5d5cb0 opkg: fix several package installation bugs
48ae44d033 opkg: gracefully handle missing $PATH, fix build warnings
1449b52f02 opkg: backport upstream fixes, code cleanups

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