Implement variable expansion on the filename field from dhcp

src/core/exec.c

@@ -95,7 +95,7 @@ int execv ( const char *command, char * const argv[] ) {
  * The expanded command line is allocated with malloc() and the caller
  * must eventually free() it.
  */
-static char * expand_command ( const char *command ) {
+char * expand_command ( const char *command ) {
 	char *expcmd;
 	char *start;
 	char *end;

src/usr/autoboot.c

@@ -33,6 +33,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <usr/dhcpmgmt.h>
 #include <usr/imgmgmt.h>
 #include <usr/autoboot.h>
+char * expand_command ( const char *command );
 
 /** @file
  *
@@ -97,7 +98,9 @@ int boot_next_server_and_filename ( struct in_addr next_server,
 		uri_encode ( filename, buf + strlen ( buf ),
 			     sizeof ( buf ) - strlen ( buf ), URI_PATH );
 		filename = buf;
-	}
+	} else { /* I don't think it could hurt the tftp case, but might as well stay out of a codepath I don't intend to rigorously test */
+                filename = expand_command(filename);
+        }
 
 	/* Download and boot image */
 	image = alloc_image();