Add qtestsign tool.this is necessary to sign the u-boot for rb3gen2 board.

qtestsign is a simple tool to "sign" ELF Qualcomm firmware
images using a dummy certificate chain ("test keys").
It partially implements the image format described in Qualcomm's
whitepaper "Secure Boot and Image Authentication"
(both v1.0 and v2.0) by extending ELF images with a
Qualcomm-specific "hash segment", consisting of:

References:
 - https://www.linaro.org/blog/initial-u-boot-release-for-qualcomm-platforms/

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>

Add u-boot-qcom recipe. This fetch from Linaro repo and build the
u-boot-qcs6490-rb3gen2.mbn ready to be flashed into rb3gen2 board
usind fastboot:

fastboot flash uefi_a ./u-boot-qcs6490-rb3gen2.mbn

Some logs:

U-Boot 2024.10-rc4-00620-ga61af132fb90 (Sep 09 2024 - 10:00:06 +0000)

Model: Qualcomm Technologies, Inc. Robotics RB3gen2
DRAM:  6 GiB (effective 5.9 GiB)
serial_msm_geni serial@994000: pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19
serial_msm_geni serial@994000: pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19
Core:  57 devices, 28 uclasses, devicetree: board

Loading Environment from nowhere... OK
In:    serial,button-kbd
Out:   serial,vidconsole
Err:   serial,vidconsole
qcom-ufshcd ufs@1d84000: Qcom UFS HC version: 4.3.0
qcom-ufshcd ufs@1d84000: [RX, TX]: gear=[3, 3], lane[2, 2], pwr[FAST MODE, FAST MODE], rate = 2
QCOM-FMP: Failed to find boot partition
Net:   No ethernet found.

scanning bus for devices...
  Device 0: (0:0) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 118871.9 MB = 116.0 GB (30431231 x 4096)
  Device 1: (0:1) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 7.9 MB = 0.0 GB (2047 x 4096)
  Device 2: (0:2) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 7.9 MB = 0.0 GB (2047 x 4096)
  Device 3: (0:3) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 31.9 MB = 0.0 GB (8191 x 4096)
  Device 4: (0:4) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 3071.9 MB = 2.9 GB (786431 x 4096)
  Device 5: (0:5) Vendor: SAMSUNG Prod.: KM2L9001CM-B518 Rev: 0700
            Type: Hard Disk
            Capacity: 31.9 MB = 0.0 GB (8191 x 4096)
starting USB...
Bus usb@a600000: Register 2000340 NbrPorts 2
Starting the controller
USB XHCI 1.10
scanning bus usb@a600000 for devices... 1 USB Device(s) found
       scanning usb for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot:  0
=> <INTERRUPT>

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>

Set u-boot-qcom as preferred virtual/bootloader.

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>

Add qcs6490-rb3gen2/sdm845-db845c u-boot configs to build boot
configurations using qcom-armv8a machine.

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>