The nrf52840-mdk-usb-dongle and pca10050 comes with a pre-flashed
bootloader (OpenBootloader).
This commit updates the boards "mpconfigboard.mk" to use DFU as
default flashing method and set the corresponding BOOTLOADER
settings such that nrf52840_open_bootloader_1.2.x.ld linker
script is used.
The default DFU flashing method can be disabled by issuing "DFU=0"
when invoking make. This will lead to "segger" being used as default
flashing tool. When using "DFU=0", the linker scripts will not
compensate for any MBR and Bootloader region being present, and might
overwrite them if they were present.
The commit also removes the custom linker script specific to
nrf52840-mdk-usb-dongle as it now points to a generic.
Updated nrf52840-mdk-usb-dongle's README.md to be more clear on
how to deploy the built firmware.
The port README.md has also been updated. In the list of target
boards a new column has been added to indicate which bootloader
is present on the target board. And for consistency, changed all
examples in the README.md to use "deploy" instead of "flash".
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