pkg: Reduce the number of times variables containing `$(shell...)` are invoked.
pkg: Reduce the number of times variables containing `$(shell...)` are invoked.

For DIRTY and RELEASED this means simply performing a static assignment with
`:=`. For HASH it is a little more complex since it is (and we want/need it to
be) a conditional assigment. However it is only used for defining TAG, so make
that a static assignment.

This reduces the number of times the complex DIRTY shell command in particular
is evaluated.

Signed-off-by: Ian Campbell <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@docker.com>

YuPengZTE/linuxkit,zx2c4/linuxkit,thebsdbox/linuxkit,JohnnyLeone/linuxkit,justincormack/linuxkit,thebsdbox/linuxkit,zx2c4/linuxkit,kencochrane/linuxkit,deitch/linuxkit,rn/linuxkit,linuxkit/linuxkit,t-koulouris/linuxkit,furious-luke/linuxkit,yankcrime/linuxkit,deitch/linuxkit,justincormack/linuxkit,linuxkit/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,zx2c4/linuxkit,JohnnyLeone/linuxkit,eyz/linuxkit,davefreitag/linuxkit,konstruktoid/linuxkit,mor1/linuxkit,ndauten/linuxkit,justincormack/linuxkit,justincormack/linuxkit,deitch/linuxkit,yankcrime/linuxkit,thebsdbox/linuxkit,linuxkit/linuxkit,rn/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,furious-luke/linuxkit,djs55/linuxkit,t-koulouris/linuxkit,t-koulouris/linuxkit,thebsdbox/linuxkit,rn/linuxkit,konstruktoid/linuxkit,kencochrane/linuxkit,kencochrane/linuxkit,ndauten/linuxkit,YuPengZTE/linuxkit,konstruktoid/linuxkit,eyz/linuxkit,thebsdbox/linuxkit,davefreitag/linuxkit,justincormack/linuxkit,eyz/linuxkit,mor1/linuxkit,deitch/linuxkit,eyz/linuxkit,linuxkit/linuxkit,radu-matei/linuxkit,YuPengZTE/linuxkit,rn/linuxkit,JohnnyLeone/linuxkit,YuPengZTE/linuxkit,yankcrime/linuxkit,kencochrane/linuxkit,deitch/linuxkit,radu-matei/linuxkit,djs55/linuxkit,t-koulouris/linuxkit,thebsdbox/linuxkit,djs55/linuxkit,yankcrime/linuxkit,radu-matei/linuxkit,davefreitag/linuxkit,t-koulouris/linuxkit,eyz/linuxkit,ndauten/linuxkit,ndauten/linuxkit,furious-luke/linuxkit,radu-matei/linuxkit,konstruktoid/linuxkit,kencochrane/linuxkit,YuPengZTE/linuxkit,ndauten/linuxkit,JohnnyLeone/linuxkit,rn/linuxkit,mor1/linuxkit,eyz/linuxkit,radu-matei/linuxkit,yankcrime/linuxkit,mor1/linuxkit,mor1/linuxkit,kencochrane/linuxkit,davefreitag/linuxkit,furious-luke/linuxkit,zx2c4/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,djs55/linuxkit,yankcrime/linuxkit,YuPengZTE/linuxkit,davefreitag/linuxkit,zx2c4/linuxkit,ndauten/linuxkit