tools/libfsimage: Fix clean and distclean make targets

tools/libfsimage: Fix clean and distclean make targets

If there is a single colon for a given target and the target
is redefined in another place (e.g. in included file) then
make executes only new target and displays following warning:

Makefile:35: warning: overriding commands for target `clean'
tools/libfsimage/common/../../../tools/libfsimage/Rules.mk:25:
warning: ignoring old commands for target `clean'

To cope with that issue define all required targets as double-colon
rules. Additionally, remove some redundant stuff.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>