remove references to u_char type, use 'unsigned char' instead

... u_char not being defined on Nuttx.
diff --git a/include/osmocom/vty/buffer.h b/include/osmocom/vty/buffer.h
index 482a919..f6c86a1 100644
--- a/include/osmocom/vty/buffer.h
+++ b/include/osmocom/vty/buffer.h
@@ -39,7 +39,7 @@
 /* Add the given data to the end of the buffer. */
 extern void buffer_put(struct buffer *, const void *, size_t);
 /* Add a single character to the end of the buffer. */
-extern void buffer_putc(struct buffer *, u_char);
+extern void buffer_putc(struct buffer *, unsigned char);
 /* Add a NUL-terminated string to the end of the buffer. */
 extern void buffer_putstr(struct buffer *, const char *);
 
diff --git a/include/osmocom/vty/command.h b/include/osmocom/vty/command.h
index 3acbe78..89dc28f 100644
--- a/include/osmocom/vty/command.h
+++ b/include/osmocom/vty/command.h
@@ -122,7 +122,7 @@
 	unsigned int cmdsize;	/*!< \brief Command index count. */
 	char *config;		/*!< \brief Configuration string */
 	vector subconfig;	/*!< \brief Sub configuration string */
-	u_char attr;		/*!< \brief Command attributes */
+	unsigned char attr;	/*!< \brief Command attributes */
 };
 
 /*! \brief Command description structure. */
diff --git a/src/vty/buffer.c b/src/vty/buffer.c
index e385f9f..def2f52 100644
--- a/src/vty/buffer.c
+++ b/src/vty/buffer.c
@@ -180,7 +180,7 @@
 }
 
 /* Insert character into the buffer. */
-void buffer_putc(struct buffer *b, u_char c)
+void buffer_putc(struct buffer *b, unsigned char c)
 {
 	buffer_put(b, &c, 1);
 }
diff --git a/src/vty/vty.c b/src/vty/vty.c
index 10a323f..f34e529 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -276,7 +276,7 @@
 			p = buf;
 
 		/* Pointer p must point out buffer. */
-		buffer_put(vty->obuf, (u_char *) p, len);
+		buffer_put(vty->obuf, (unsigned char *) p, len);
 
 		/* If p is not different with buf, it is allocated buffer.  */
 		if (p != buf)