boot/boot_serial: allow nonaligned last chunk

The last data packet might be non aligned to multiple of
the flash write-block-size. This cause that the `image upload`
mcumgr command mighty fail if the device flash driver doesn't
support one-byte write-block-size.

This patch complements the last write operation to aligned chunk
so it meet the requirements.

Added check for ensure than received data chunk doesn't
cross expected image size.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>