Support headless configuration on Raspberry Pi

The goal of the headless configuration setup with RPI is to make it easy for users to enable things like ssh and set wifi credentials easily. It uses files created in the FAT partition to trigger enablement of certain features. Since it uses the FAT partition it is easy to access this on any system.

Currently, we mount the bootloader FAT partition in /uboot, while Raspberry PI OS related utilities expect it at /boot, hence the following services do not work as intended:

• lib/systemd/system/apply_noobs_os_config.service
• lib/systemd/system/raspberrypi-net-mods.service
• lib/systemd/system/sshswitch.service

Acceptance criteria:

• the above scripts are modified to expect the boot partition at /uboot instead of /boot
• mender-docs are updated to reflect that headless configuration is supported