samples: flash_shell: unaligned and pattern write

Adds unaligned and pattern write functionality.
The reason this was added is chips with large pages, e.g. 256 can't be
normaly writen via shell, thus the need to write a pattern with any
length to a memory area.
The unaligned write command allows writing a data frament in any place
in memory, the data backup, protection, erasing and writing is
automaticaly handled. This makes it easier to test all flash controller
functionality in one command, also write any persistent data to any
flash address.

Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>