linux-minnow: Fix to handle multiple sources sha256 and md5sum

There are two different souces for rt and non-rt kernel. Hence
with one md5sum and sha256 the build breaks. This change will
help to handle multiple sources and their md5sum and sha256
separately

JIRA: SB-3226

Signed-off-by: Sujith H <Sujith_Haridasan@mentor.com>