crossgcc: Re-download the archive if it is incomplete

If the buildgcc is interrupt by Ctrl-C, probably part of
an archive is downloaded. If we run buildgcc again, the
incomplete archive would be considered as cached file
and skipped.

We check file hashes to see if the file is complete. If test
is failed, we need to delete the partially-downloaded file
and download it again.

sha1sum is quite different among the distributions.
Only Linux, Cygwin, Darwin have been tested.

Once new archive is deployed, a new checksum would be created,
which should be uploaded along with the script buildgcc.

Change-Id: Ibb1aa25a0374f774e1e643fe5e698de7bf7cc418
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/4511
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/crossgcc/sum/gcc-4.9.2.tar.bz2.cksum

@@ -0,0 +1 @@
+79dbcb09f44232822460d80b033c962c0237c6d8  tarballs/gcc-4.9.2.tar.bz2