Update the usage of shallow_flash.sh for MAC user.

Fix #213
Merge pull request #219 from askeing/add_mac_usage_for_shallow_flash

Update the usage of shallow_flash.sh for MAC user.
Merge pull request #220 from askeing/master

update the desktop b2g download link
Update Help Menu and Add 2.1.0
Issue #221 - Fix downloading tbpl builds with build id
Merge pull request #222 from bkerensa/patch-1

Update Help Menu and Add 2.1.0
Merge pull request #223 from zacc/issue_221

Issue #221 - Fix downloading tbpl builds with build id
Merge pull request #228 from askeing/master

End of maintenance of old version pvt flash tool.
Change to lower case because of consistency
Merge pull request #230 from echangmoz/master

Change to lower case because of consistency