firmware: fix directory creation rule matching with make 3.80

Since make 3.80 doesn't support secondary expansion it uses a fallback
rule to create firmware directories which is matched after primary
expansion of the $(installed-fw) rule's prerequisite. Commit
6c7080a6

 [firmware: fix directory creation rule matching with make
3.82] changed the expression generated after primary expansion such
that the fallback was not matched. Updating the fallback rule to match
the new look primary expansion is not an option for various reasons.

The trailing slash added here to $(INSTALL_FW_PATH)/. while defining
installed-fw-dirs fixes builds with make 3.82 since this will provide
a matching rule for $(INSTALL_FW_PATH)/$$(dir %) when % is in the base
firmware directory (ie. $(dir %) gives './'). Versions of make prior
to 3.82 will strip this trailing slash along with the one generated by
$(dir %) when % is in the base firmware directory and as such continue
to function as before.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Tested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>