install-buildtools: bump default to yocto-3.1_M3, fixes

Add ability to check md5sum (yocto-3.1_M2 and before) or sha256
(yocto-3.1_M3 and beyond).

Make regex for path in checksum file optional, since
for yocto-3.1_M3 the format is <checksum>  <filename>,
but prior releases was <checksum>  <path><filename>

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>