Mbed OS builds .bin images by default for most targets, but many
targets (e.g. ST targets running the latest ST-LINK firmware) also
support .hex files, making this demo much easier to prepare. In
case a target doesn't support .hex files, README.md already contains
steps for merging .hex files into a single one and converting it to
a .bin file.

The majority of flash devices support reading single bytes without
alignment concerns. The config `mcuboot.read-granularity` enables
additional code to handle read alignment, and it comes with a slight
code size and performance overhead.

This matches the responding change in mbed-mcuboot-blinky.

The latest Mbed OS already has `qspif.QSPI_MIN_PROG_SIZE` set to 4
for MCU_NRF52840 which is the base target of NRF52840_DK and EP_AGORA.

Co-authored-by: Lingkai Dong <[email protected]>

…-demo