mkrepo.sh: make sure Packages files are properly sorted

Works around the base system installation failure (the error happens only
with Ubuntu 14.04). Note that the Packages files on Debian/Ubuntu mirrors
are sorted (alphabetically).

Change-Id: I20262e14b90010a67e7fc1d16843313481d0d6a7
Merge "mkrepo.sh: make sure Packages files are properly sorted"