Fix uart/shell conflict

Currently there are to macros: ``UART0_BUFSIZE`` and ``SHELL_BUFFER_SIZE``. This is problematic in a number of points:
a) ``UART0_BUFSIZE`` is very because of the possibility of memory constraints on certain boards very small.
b) currently ``SHELL_BUFFER_SIZE`` is at 127 bytes while ``SHELL_BUFFER_SIZE`` is at 32 bytes. This leads to weird behaviour with shell entry longer than the given 32 byte (the front of the input string is cut of).
c) The shell can be used over different media than UART theoretically (this is not a problem in the current setup but would be if we e.g. just set ``SHELL_BUFFER_SIZE`` to ``UART0_BUFSIZE``).

Therefore, I propose to make ``UART0_BUFSIZE`` configurable in ``cpu-conf.h`` and to make the buffer size of the shell a variable property of a shell (so one can set it e. g. to UART0_BUFSIZE if a UART shell is used or any other value they might seem fit).

Also consider my PR in projects: