Explamantions of common env vars:

$TARGET is target device, duet, kevin or other
$ROOTFS represents target linux rootfs, Cadmium is made to be independent of rootfs
$CADMIUMROOT is root directory of Cadmium containing board/, kernel/, fs/, tmp/ etc. directories

Various hacks that were used:

/dev/mmcblk?: On Duet, Linux kernel randomly switches eMMC device between mmcblk0 and mmcblk1, therefore a wildcard has to be used
eMMC installation script has been split in 3 parts:
install-to-emmc-begin, which sets up partitioning, firmware and kernel
install-to-emmc-middle, which installs rootfs. this should be as small as possible
install-to-emmc-end, which sets up rootfs in order to make it easier to use(user accounts and cadmium-specific configs)
kernel/build has duet-specific kernel source :(

board/$TARGET/ is for stuff specific to one device, usually: config files, dts configuration, kernel patches and firmware
fs/ is for building rootfs that can be used on target device
fs/$ROOTFS is for parts that are rootfs-specific
kernel/ contains only kernel-specific files, including scripts to test whether built kernel works or not
bootfw/ has scripts to set up kernel and partition layout

BASEBOARD is "reference" board for family of chromebooks or common ancestor
BOARD is the target board, for example: kevin, krane
TYPE is type of device, laptop, tablet, convertible-laptop is laptop with 360° hinge, convertible-tablet is tablet with keyboard attachment or handheld

ARCH uses kernel's naming, arm, arm64 etc.
ARCH_UNAME is what appears in uname -m, armv7l, aarch64 etc.
ARCH_DEB is whatever debian calls the architecture of device, armhf, arm64
YES ALL THIS ARCH STUFF IS A GIANT MESS
CROSS_COMPILE is self-explanatory
SOC - self explanatory
BOOTFW is boot firmware of device, chromebooks use depthcharge