da1469x: Specify custom location of da1469x_flash_loader

[jrotkiewicz]

User may now use the mynewt value FLASH_LOADER_TARGET in bootloader target to specify the da1469x_flash_loader target location.

[jrotkiewicz]

@kasjer

[andrzej-kaczmarek]

BSP is definitely not a good place to have this configured - it should be in some common package for MCU as otherwise syscfg has to be defined for each BSP separately

also I am not sure if syscfg is the way to go here, I'd lean towards using env variable to overwrite flash loader target and/or .elf location

[apache-mynewt-bot]

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