minifs: Let board files declare packages

The PACKAGES variable was cleared after the board scripts had loaded,
meaning that they could not declare their own packages.
Now fixed.

Signed-off-by: Michel Pollet <buserror@gmail.com>
zedboard: Add the zbootgen tool

Now download the tool that generate the zynq boot stream, and
create one with the u-boot that was previously compiled

Signed-off-by: Michel Pollet <buserror@gmail.com>