manual: github: merge tag/commit ID examples

However, it is possible to download tarballs directly from the repository

on github.

If the package version matches a tag, then this tag should be used to

identify the version:

------------------------

FOO_VERSION = v1.0

FOO_SITE = http://github.com/<user>/<package>/tarball/$(FOO_VERSION)

------------------------

If the package has no release version, or its version cannot be

identified using tag, then the SHA1 of the particular commit should be

used to identify the version (the first 7 characters of the SHA1 are

enough):

------------------------

FOO_VERSION = 1234567

FOO_VERSION = v1.0 # tag or (abbreviated) commit ID

FOO_SITE = http://github.com/<user>/<package>/tarball/$(FOO_VERSION)

------------------------

Note that the name of the tarball is the default +foo-1234567.tar.gz+

.Notes

- The FOO_VERSION can either be a tag or a commit ID.

- The tarball name generated by github matches the default one from

  Buildroot (e.g.: +foo-1234567.tar.gz+),

  so it is not necessary to specify it in the +.mk+ file.

- When using a commit ID as version, usually the first 7 characters of

  the SHA1 are enough.