fix(sys-kernel/coreos-kernel): Disable outoftree for patched versions.

Missed this in 59d86fde1c2fa477b91f76d29ea1da145ca6f618, we don't want
to patch shared source trees and doing so causes a sandbox violation. So
disable CROS_WORKON_OUTOFTREE_BUILD for versions with the no-firmware
patch.
Merge pull request #109 from marineam/kernel

fix(sys-kernel/coreos-kernel): Disable outoftree for patched versions.