Switch manifest to prcxuying/b2g-manifest

 2017  git clone https://github.com/prcxuying/B2G
 2018  ls
 2019  cd B2G/
 2020  ls
 2021  BRANCH=v1.2 ./config.sh fugu
...
Get git://github.com/mozilla-b2g/b2g-manifest
remote: Counting objects: 1050, done.
remote: Compressing objects: 100% (600/600), done.
remote: Total 1050 (delta 685), reused 770 (delta 431)
Receiving objects: 100% (1050/1050), 219.44 KiB | 23.00 KiB/s, done.
Resolving deltas: 100% (685/685), done.
From git://github.com/mozilla-b2g/b2g-manifest
 * [new branch]      master     -> origin/master
 * [new branch]      v1-train   -> origin/v1-train
 * [new branch]      v1.0.0     -> origin/v1.0.0
 * [new branch]      v1.0.1     -> origin/v1.0.1
 * [new branch]      v1.1.0hd   -> origin/v1.1.0hd
 * [new branch]      v1.2       -> origin/v1.2
 * [new tag]         B2G_1_0_1_20130213094222 -> B2G_1_0_1_20130213094222
 * [new tag]         B2G_1_1_0_hd_20130530182315 -> B2G_1_1_0_hd_20130530182315
 * [new tag]         B2G_1_1_0_hd_20130530182315_BASE -> B2G_1_1_0_hd_20130530182315_BASE
 * [new tag]         closing-nightly -> closing-nightly
fatal: manifest 'fugu.xml' not available
fatal: manifest fugu.xml not found
Repo sync failed