prebuilt.py: fix variant detection regex
prebuilt.py: fix variant detection regex

Without this, I get:

Traceback (most recent call last):
  File "./prebuilt.py", line 495, in <module>
    main()
  File "./prebuilt.py", line 491, in main
    git_sync=options.git_sync)
  File "./prebuilt.py", line 408, in UploadPrebuilt
    git_file = os.path.join(build_path, DetermineMakeConfFile(board))
  File "./prebuilt.py", line 378, in DetermineMakeConfFile
    raise UnknownBoardFormat('Unknown format: %s' % target)
__main__.UnknownBoardFormat: Unknown format: tegra2_seaboard

Change-Id: I1d595b7e1c596139acca344f7a145143686dace1

BUG=chromium-os:8388
TEST=./prebuilt.py  -p ~/chromiumos -b tegra2_seaboard  -u user@workstation:/tmp

Review URL: http://codereview.chromium.org/4184007

fivethreeo/scripts,bcwaldon/coreos-scripts,endocode/scripts,crawford/scripts,marineam/coreos-scripts,andrejro/coreos--scripts,endocode/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,trnubo/scripts,trnubo/scripts,bcwaldon/coreos-scripts,crawford/scripts,sigma/coreos-scripts,nicescale/scripts,mjg59/scripts,nicescale/scripts,gtank/scripts,smilart/scripts,glevand/coreos--scripts,sigma/coreos-scripts,gtank/scripts,crawford/scripts,coreos/scripts,smilart/scripts,nicescale/scripts,BugRoger/scripts,fivethreeo/scripts,marineam/coreos-scripts,vmware/coreos-scripts,gtank/scripts,mischief/scripts,vmware/coreos-scripts,mjg59/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,vmware/coreos-scripts,zhang0137/scripts,trnubo/scripts,mischief/scripts,zhang0137/scripts,zhang0137/scripts,mischief/scripts,BugRoger/scripts,endocode/scripts,coreos/scripts,glevand/coreos--scripts,fivethreeo/scripts,mjg59/scripts,smilart/scripts,marineam/coreos-scripts,sigma/coreos-scripts