Change makefile target to archive tar package for bootstrap project

When you using the command $ make tarball, which generate a tar package
named after AT91Bootstrap-$(VERSION)$(REV)$(SCMINFO).tar.gz

The package contains a file with the commit id which display in the booting log.
This information is helpful to identify the precise version.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>