system: add option to pass extra args to post-build and post-image scripts

It can be useful to have different configuration use the same post-build
and/or post-image scripts as they share a common infrastructure, but yet
have minor differentiation.

This option allows passing zero or more additional arguments to each
post-build or post-image script.

The same set of extra arguments are passed to all scripts, it is not
possible to pass different arguments to each script.

[Peter: fix help text, post-image gets called with the images dir]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>