libc: string: added a string maniplation module.

lib/string contains the '<string.h>' part of libc. These functions are
all, in some way, relevant to C style strings (char *). The functions
implemented in v1 are:

	* strcmp (compare 2 strings)
	* strchr (find first occurance of a character)
	* strlen (find the length of a string)
	* strnlen (see if a string is longer than a given amount)

The return value of strnlen is the value of strlen if the string is NOT
longer than the given amount, if it is the given length is returned.

Signed-off-by: Michel Megens <dev@michelmegens.net>
make: mrproper: added generated include dirs.

Added the directories include/config and include/generated to the
mrproper target for removal. Invoking `make mrpoper' will now also first
do a regular clean before removing all extra 'mrproper files'.

Signed-off-by: Michel Megens <dev@michelmegens.net>