etherboot: [build] sort objects in blib.a

etherboot: [build] sort objects in blib.a

When building hvmloader for Xen tools the ipxe objects are also linked
into the binary. Unfortunately the linker will place them in the order
found in the archive. Since this order is random the resulting hvmloader
binary differs when it was built from identical sources but on different
build hosts. To help with creating a reproducible binary the elements in
blib.a must simply be sorted before passing them to $(AR).

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>