Introduce new testing-app example.
Add a new coap error code for blockwise transfer.
Add include-file for bootloader callbacks (jumptable).
Note that only the bootloader for osd-merkur-256 will support
OTA-update, the -128 simply has not enough flash memory, so only
in the -256 we have the bootloader functions in the jump-table
of the bootloader and in the bootloader-if.h include-file.
