WIP: raspberry2/3: Fix boot issue caused due to kernel7.img

The newer Raspberry Pi 3 firmware package from Debian contains kernel7.img and it is preferred over kernel.img that is being copied from u-boot currently. This leads to unexpected rules and boot failure. Fix this by copying u-boot as kernel7.img as well.

This change is being proposed as a result tests done by @njoseph-guest which concluded the kernel7.img that is present in rasp-firmware package is inadvertently being copied into our /boot/firmware partition. Removing the file or copying kernel.img as kernel7.img resulted in a proper fix. This patch is marked as work-in-progress as it has not been tested fully on raspberry pi 2 and 3 with freshly built images. This following needs to be tested:

• Build images for Raspberry Pi 2 and ensure that devices are available on network after boot (barring the issue the we are facing which forces us to reboot after waiting 5 minutes to get things properly working).
• Repeat for Raspberry Pi 3.

@mray-guest We believe this should solve the issue you were facing with when you built Raspberry Pi images.