boot-wrapper-aarch64: new package

The boot wrapper contains both a minimal bootloader and a tool to
generate an image suitable for execution by the AArch64 software
simulator. The image generated embeds the minimal bootloader, the
kernel image, the Device Tree Blob and the kernel command line.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>