Disabled per-feed package directories

The option CONFG_PER_FEED_REPO is available with make menuconfig:

Image configuration -> Separate feed repositories

If disabled, one bin/<target>/packages directory is created
instead of additional subdirs for different feeds.
Enabled wget with SSL and ca-certificates package

Without this, we cannot host release ipk files at
github, as its URL is

https://github.com/m-creations/openwrt/releases/download/
Added version number to be embedded in opkg URL
Added documentation about the steps to upload a release to github