Debian RPi3 image currently utilizes files from the boot partition of generic RPi. These are available in the RPi firmware repo. The boot subtree content only takes about 22 MB. But to get there, git remote add has to first bring down 8 GB of Git objects.

This is way too much overhead; it should not be performed on a regular basis. A lighter alternative is sought.

Steps to set up sparse checkout:
PIFIRMREPO=https://github.com/raspberrypi/firmware.git
cd ${TARGETDIR}
git init
git remote add -f origin ${PIFIRMREPO}

Example report of above operations:
git remote add -f origin https://github.com/raspberrypi/firmware.git
Updating origin
remote: Counting objects: 704913, done.
remote: Total 704913 (delta 0), reused 0 (delta 0), pack-reused 704913
Receiving objects: 100% (704913/704913), 7.87 GiB | 3.46 MiB/s, done.
Resolving deltas: 100% (558580/558580), done.
At this point, 8+ GB of disk space have been consumed.

To do the sparse checkout:
git config core.sparseCheckout true
echo boot >> .git/info/sparse-checkout
echo git pull --depth=4 origin master
Result:
remote: Counting objects: 7620, done.
remote: Compressing objects: 100% (4171/4171), done.
remote: Total 7620 (delta 4577), reused 4106 (delta 2813), pack-reused 0
Receiving objects: 100% (7620/7620), 54.71 MiB | 2.74 MiB/s, done.
Resolving deltas: 100% (4577/4577), completed with 6 local objects.
This gets a boot directory of about 22 MB.

vcgencmd is need to gather/confirm certain types of info about the Broadcom CPU chip. In the standard Raspbian image, it resides at: /opt/vc/bin/vcgencmd
This was discovered while migrating rootfs from microSD to USB flash; used to confirm that boot will take place from USB instead of microSD.

More info at: https://www.elinux.org/RPI_vcgencmd_usage