sys/shell: Exit the shell on ctrl-D

Right now the only way to exit the shell is if stdin is closed. This
works on native, but on an embedded platform stdin is the uart and thus
is never closed.

This patch causes the shell loop to exit on EOT (ASCII 0x04 / ctrl-D),
also called "End-of-Transmission".

sys/shell/shell.c

@@ -33,6 +33,8 @@
 #include "shell_commands.h"
 
 #define ETX '\x03'  /** ASCII "End-of-Text", or ctrl-C */
+#define EOT '\x04'  /** ASCII "End-of-Transmission", or ctrl-D */
+
 #if !defined(SHELL_NO_ECHO) || !defined(SHELL_NO_PROMPT)
 #ifdef MODULE_NEWLIB
 /* use local copy of putchar, as it seems to be inlined,
@@ -235,7 +237,7 @@ static int readline(char *buf, size_t size)
         }
 
         int c = getchar();
-        if (c < 0) {
+        if (c < 0 || c == EOT) {
             return EOF;
         }