fs/common.mk: delay evaluation of variables

This makes things easier to understand and more consistent with the pkg-infra.
For example, it removes the need for '$$@' in the CMD variables of fs/*/*.mk.

It also makes it possible to update the variables later, e.g. in the package
override file.

It also makes sure that the date will be recorded correctly in Yann E. Morin's
patch that logs the MESSAGE macros to a file.

The fs/*/*.mk must be updated as well because the '$@' shouldn't be quoted
anymore in the CMD variables or the hooks.

The $(eval ...) for the dependencies is redundant, because the $(ROOTFS_TARGET)
variable is already eval'd. Note that it is only redundant if the evaluation of
the uses of the variable is also delayed.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>