cros_workon: redefine the concept of a "workon" package list to depend on the board
cros_workon: redefine the concept of a "workon" package list to depend on the board

* Modified all workon listing functions to also look for keyword

* Added a fallback to list all workon ebuilds if keyword is not specified, which
is needed for cros_mark_all_as_stable, which does not differentiate between boards.

This, amongst other potential issues, resolves the case when it was possible to
start working on a package not keyworded for the given board, and making
build_packages fail unconditionally.

TEST=below
$ ./cros_workon list --all --board=x86-generic |wc -l
73
$ ./cros_workon list --all --host |wc -l
57
Looking at the lists rather than "|wc -l" looks correct
$ ./cros_mark_all_as_stable
^ Produces satisfactory result

BUG=6700

Change-Id: Ieee92a39febcef5fb95e59cf97b6e63281a7c750

Review URL: http://codereview.chromium.org/3400001

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