Integrate mcuboot in framework-zephyr when using CONFIG_BOOTLOADER_MCUBOOT

[shlomozippel]

When CONFIG_BOOTLOADER_MCUBOOT is enabled it would be really nice to have pio targets for bootloader stuff

1. buildbootloader to build mcuboot with the same BOARD settings
2. uploadbootloader to flash the bootloader image
3. Use imgtool to sign / encrypt the target firmware

Currently it seems like the only thing that happens is the program start address is changed to slot0 partition and thats it. I noticed mcuboot is fetched into framework-zephyr/_pio/bootloader but there really is no way to build it or upload it

Note: this is quite similar to this nrf issue: platformio/platform-nordicnrf52#150