Size of build image for mcuboot with serial or usb DFU enabled?

[BrianW]

Working on a nrf5340 project, and running into limits of the size of the flash to store my application image along with mcuboot bootloader.

I had a mucboot setup that fitted into 56Kb (signed images, secondary slot on external flash). This did NOT include the serial DFU however, and (of course) one of my first units out in the field got bricked during a DFU (using dfu_target lib to update the secondary slot from a file loaded on USB FAT FS mounted on the external flash... not yet got the unit to see what got corrupted...)

I expose the USB interface, so would like to enable the serial DFU functionality in mcuboot so that at least I have a method to DFU even if my main app image gets corrupted...Some questions to get this to work:

1/ The buttons on the device are via a gpio expander on the i2c interface, it seems mcuboot likes a 'dfu button' to be a direct gpio (at least if I set the alias:

mcuboot-button0 = &button2;

then it doesn't build because button2 is 

button2: button_2 {

            gpios = <&ioexp0 4 (GPIO_ACTIVE_LOW)>;    // GPA4

). Is this correct or should it work via a ioexpander?

BTW This is not a deal breaker as I see I can have a 'DFU delay' at boot to always check if the dfu is to be activated on the serial/USB

2/ serial DFU (virtual com port on USB) vs USB DFU : which is better to use? Which gives me a smaller mcuboot image?

3/ Image size : mcuboot with DFU_USB is now 76+Kb

Is this normal? Are there ways to slim this down? also, the mcuboot child_image build has its own mcuboot.conf - does this addon to the usual prj.conf, or replace it completely?

The image size is an issue because my main app uses https over wifi networking (nrf7002) and the wifi code+networking+TLS takes up a LOT of space....

thanks for any tips to get a solid mcuboot in the least flash space!