A new CLI subcommand was added, flash, which behaves very similar to the already present compile CLI comamnd, but with the added ability to target a bootloader. The command is used like so: qmk flash [-h] [-b] [-kb KEYBOARD] [-km KEYMAP] [-bl BOOTLOADER] [filename].
A -kb <keyboard> and -km <keymap> is expected, or a configurator export JSON filename. A bootloader can be specified using -bl <target>, and if left unspecified, the target is assumed to be :flash. -bl can be used to list the available bootloaders.
If -km <keymap> is provided, but no -kb <keyboard>, then a message is printed suggesting the user to run qmk list_keyboards.
Zach White