opkg: bump to version 2017-12-08

opkg: bump to version 2017-12-08

This updates package to the latest commit from the lede-17.01 branch. It
contains few fixes backported from the master:
1) SHA256 fix
2) URL encoding which allows hosting packages on some more picky servers

Changes:
9f61f7a opkg_download: decode file:/ URLs
3c46c88 file_util: implement urldecode_path()
79908c2 file_util: consolidate hex/unhex routines
793fbac opkg: encode archive filenames while constructing download URLs
a6bb5cb file_util: implement urlencode_path() helper
098e774 libopkg: fix SHA256 calculation for big endian system

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>