[libc] Add isxdigit()

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ctype.h

 #define isdigit(c)	((c) >= '0' && (c) <= '9')
 #define islower(c)	((c) >= 'a' && (c) <= 'z')
 #define isupper(c)	((c) >= 'A' && (c) <= 'Z')
+#define isxdigit(c)	(isdigit(c) || ((c) >= 'A' && (c) <= 'F') || ((c) >= 'a' && (c) <= 'f'))
 
 static inline unsigned char tolower(unsigned char c)
 {