Uboot qemuarm64 improvements

[vasilvas99]

Description

1. Add bootmenu for QEMUARM64 (uboot) that allows the user to short-circuit the rauc slot counting and directly boot one of the images:

2. Remove the qemuarm64.cfg as it's redundant now.

3. Add fallback_bootargs (empty) for when fdt fails to obtain the bootargs from the qemu "hardware"

Notes on behavior:

1. The user is given 3 seconds to press a key before uboot proceeds with the normal boot process (Autoboot)
2. The Boot SDV Image Full and Boot SDV Image Minimal change the bootargs at the end of the boot script, thus booting from the specified image and setting the rauc slot, but they do not change the boot order. This variable is still managed by rauc from userspace.
3. From 3) => If the last automated boot was from slot SDV_A, the user booted manually with Boot SDV Image Minimal (SLOT_B) from the uboot menu and restarts, the autoboot will still be from slot SDV_A.
4. Reset and Power off are convenient fast reset/shutdown since the kernel is not yet booted at this point.