pointercal-xinput: allow package to be empty for MinnowBoard

Allow pointercal-xinput package to be empty so that required
ipk is generated for ade to build successfully.

JIRA: http://jira.alm.mentorg.com:8080/browse/SB-3629

Signed-off-by: Yasir-Khan <yasir_khan@mentor.com>
Merge pull request #330 from Yasir-Khan/minnow-sato-ade

pointercal-xinput: allow package to be empty for MinnowBoard