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.
When a client sends a CoAP request with no block2 size,
the default one would be set to REST_MAX_CHUNK_SIZE.
However, this is not guaranteed to be a power of 2.
This can lead to clients receiving a bigger payload than expected as
part of the header, and ending up with duplicated content.
Setting the default to COAP_MAX_BLOCK_SIZE,
which is guaranteed to be a power of 2, fixes this.
