BOOTPROT defines the size of the boot loader in bytes. The defined boot loader section is write protected. So to (over)write the bootloader or write a programme without using a bootloader, this value has to be set to 0x7 which leads to a bootloader size of 0 bytes.
On the Feather M0 basic proto boards I used the BOOTPROT value has been set to 0x2 which results in the first 8k of the flash memory to be writeprotected.
The possible values are described in the SAMD21 datasheet in chapter 22.6.5. NVM User Configuration
Adafruit Feather M0 Basic Proto – ATSAMD21 Cortex M0: At the Feather M0’s heart is an ATSAMD21G18 ARM Cortex M0 processor, clocked at 48 MHz and at 3.3V logic, the same one used in the new Arduino Zero. This chip has a whopping 256K of FLASH (8x more than the Atmega328 or 32u4) and 32K of RAM (16x as much)! This chip comes with built in USB so it has USB-to-Serial program & debug capability built in with no need for an FTDI-like chip. Read more.
Make a robot friend with Adafruit’s CRICKIT – A Creative Robotics & Interactive Construction Kit. It’s an add-on to our popular Circuit Playground Express, FEATHER and other platforms to make and program robots with CircuitPython, MakeCode, and Arduino. Start controlling motors, servos, solenoids. You also get signal pins, capacitive touch sensors, a NeoPixel driver and amplified speaker output. It complements & extends your boards so you can still use all the goodies on the microcontroller, now you have a robotics playground as well.