Add Debian packaging

with arm-trusted-firmware!1 (merged) this builds a working uboot for the devktit for me.

Will unwip once

• tested on the phone
• i can add ci (depends on atf MR, see below)

his allows to update from userspace via

u-boot-install-librem5 /dev/mmcblk0

Builds via

DEB_BUILD_PROFILES="pkg.uboot.subarch.imx" dpkg-buildpackage -uc -us -B --host-arch=arm64

I can't add CI before arm-trusted-firmware!1 (merged) is merged.

Closes: image-builder#3 (closed), #20 (closed)