pie: piegen -- Only copy required sections

When building the blob in the generated header file, we may
shrink the output blobk and only copy the sections with the SHF_ALLOC
bit set, the other ones are not needed at runtime.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>