version command: make it scriptable

version command: make it scriptable

you can now set a variable in a script like set version [version].

Also version takes an optional argument "git" to show git version
of source. If git is not installed during the build, then this
will yield an error that is ignored during the build and "version git"
returns an empty string.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>