To boot over USB, simply run
make boot usb=sdc
sdc is the device node for the USB disk itself (not a partition like sdc2).
The OS image (.iso file) will be written to that USB drive.
On WSL or other host environments where
/dev device nodes don't exist, you can simply run
make iso and burn the
.iso file in the
build/ directory to a USB drive.
For example, on Windows we recommend using Rufus to burn ISOs.
Then, once the bootable USB is ready, plug it into your PC, restart or power on the machine, and choose that USB device from the BIOS or legacy boot device screen.