[QEMU] add support for qemuarm and qemuarm64

Add support to build Edgehog Things for QEMU virt ARMv7/8 with poky BSP.

• build U-boot (which is not built by default for QEMU)
• build initramfs
• add WKS template file for QEMU
• exclude grub and u-boot-seco* from build (uses upstream u-boot)
• load u-boot environment from virtio block device
• build boot.scr in a dedicated recipe (u-boot-default-script-rpi renamed to u-boot-default-script-edgehog) - the other boards build the script in the u-boot-seco* recipe
• add fw_env configuration (like for raspberrypi, the U-boot environment is in the uboot.env file in the first FAT partition)
• add rauc configuration

Command to run the image in qemu:

BIOS=<path/to/u-boot.bin> runqemu <path/to/qemuboot.conf> slirp nographic