Bug 847351 - Avoid multiplication of definitions in B2G/.repo/manifests/...

Move all remotes into a single file base_remotes.xml and include it
in all manifests.

Apart from the mechanical move from each manifest file to base_remotes.xml, the following changes were made:

* <remote name="b2g" fetch="https://git.mozilla.org/b2g"/> is replaced with
  <remote name="b2g" fetch="git://github.com/mozilla-b2g/"/>
* the remote "b2ggithub" is deleted and "b2g" used instead
FILES: emulator.xml
WHY: To unify with other manifests. I believe https://git.mozilla.org/b2g and git://github.com/mozilla-b2g have identical contents. People can create pull requests on github, so I picked up github.com.

* <remote name="linaro" fetch="http://android.git.linaro.org/git-ro/"/> is replaced with
  <remote name="linaro" fetch="git://android.git.linaro.org/"/>
FILES: galaxy-nexus.xml, galaxy-s2.xml, nexus-s-4g.xml, nexus-s.xml, optimus-l5.xml, pandaboard.xml
WHY: To unify with other manifests. Prefer the git:// over http://

* removed unused remote "github"
FILES: galaxy-nexus.xml, galaxy-s2.xml, hamachi.xml, keon.xml, leo.xml, nexus-s-4g.xml, nexus-s.xml, optimus-l5.xml, tara.xml
WHY: Unused

* <remote name="mozillaorg" fetch="http://git.mozilla.org/"/> is replaced with
  <remote name="mozillaorg" fetch="https://git.mozilla.org/releases"/>
  and the corresponding adjustments in affected projects (ie name="releases/gaia.git" -> name="gaia.git")
FILES: keon.xml, peak.xml
WHY: To unify with other manifests. Prefer the https:// over http://

* removed unused remote "b2gmozilla"
FILES: otoro.xml
WHY: Unused

* <remote name="b2g" fetch="https://github.com/mozilla-b2g/"/> is replaced with
  <remote name="b2g" fetch="git://github.com/mozilla-b2g/"/>
FILES: tara.xml
WHY: To unify with other manifests.