firmware/shim: better filtering of dependency files during Xen tree setup

firmware/shim: better filtering of dependency files during Xen tree setup

I have no idea what *.d1 is supposed to refer to - we only have .*.d
and .*.d2 files (note also the leading dot). Also switch to passing
-name instead of -path to find - that's a requirement for .*.d et al to
work, but would probably have been better from the beginning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>