tools: Revert c/s 25150:b490ef93bad7 tools/libfsimage: include Rules.mk first

tools: Revert c/s 25150:b490ef93bad7 tools/libfsimage: include Rules.mk first

tools/libfsimage/Rules.mk relies on having certain variables set already; if
they're not set, the definitions dont' work right.  The result was a bunch
of empty files and pygrub failing with an uninformative error message.

It's likely that this didn't cause anyone problems becasue changing the
Makefiles didn't cause a re-build; building from a fresh repo results in
completely empty filesystem plugin binaries.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>