wgtpkg-zip: use PATH for retriving programs

When using yocto, the directive 'find_program'
of cmake returns a path that matches the host
path when builing, not the path of the file on
the target.

Using the standard algorithm for seeking
programs using PATH should solve the problem.

Change-Id: I426a88fe044a1d1b3a72124da17ca8a188caad9c
Signed-off-by: José Bollo <jose.bollo@iot.bzh>