Honour XEN_LINUX_GIT_LOCALBRANCH and XEN_LINUX_GITREV variables

Honour XEN_LINUX_GIT_LOCALBRANCH and XEN_LINUX_GITREV variables

XEN_GIT_ORIGIN to set the name of the remote origin; the default is
still "xen".

XEN_LINUX_GIT_LOCALBRANCH allows the name of the local branch to be
set.  The default is the same as the remote branch.

XEN_LINUX_GITREV allows a specific GIT revision to be checked out,
rather than a branch.  By default it is the remote branch name.

This change should result in no change in behaviour in the normal
case.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>