vars: Use fbpacktool for unpacking bootloader.img on Tensor pixels

All Pixels modem.img: fbpack version 1
QCOM Pixels bootloader.img: fbpack version 1
Tensor Pixels bootloader.img: fbpack version 2

Test:
export WORK_DIR=/some/dir/with/factory/images
source vendor/lineage/vars/pixels
for d in ${devices[@]}; do lineage/scripts/pixel/firmware.sh $d; done
for d in ${devices[@]}; do git -C vendor/google/$d status; done

Co-authored-by: Chirayu Desai <chirayudesai1@gmail.com>
Change-Id: Ic1990f75a6be515e5d91d65e3ddf88b8e0c90001