Add mkbootimg package

The mkbootimg utility will be used to build the kernel boot images
for the projects whose bootloaders expect Android-style boot images.

Previously it was expected that a pre-built mkbootimg is provided in
the root of the kernel source tree. But that lets to build the
OpenELEC for such projects only on the platform where the mkbootimg
was built.

Closes #11
[k200] Run broadcom-wifi.service after systemd-udevd.service to ensure proper Wi-Fi initialization
Add a possibility to create the projects for devices that could have
multiple boards or reference designs

Such projects should be compiled differently for each board.
The board for multi-board projects must be specified in the
command line, e.g: ARCH=arm PROJECT=Amlogic BOARD=g18 make
[aml] Create single 'multi-board' project for all Amlogic devices, instead of Meson6-G18 and Meson8-K200
[k200] Make broadcom-wifi.service require the systemd-udevd.service to ensure proper Wi-Fi initialization
[aml] Specify the CPU scaling governor, min and max frequency in the kernel command line, and set them in platform_init script
Revert "Add a possibility to create the projects for devices that could have"

This reverts commit bff8377721bb131a2bf99aa244d9e03b0e19efa9.
Add a possibility to create the projects that could have several different devices

Such projects should be compiled differently for each device.
A device for multi-device projects must be specified in the
command line, e.g: ARCH=arm PROJECT=Amlogic DEVICE=m8 make
[aml] Convert the Amlogic project to be multi-device