TFTP: add host ip addr support

allow to use a different server as set in serverip
add CONFIG_TFTP_FILE_NAME_MAX_LEN to configure the file name length
if not defined the max length will be at 128

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/cmd_net.c b/common/cmd_net.c
index 56eb684..dbf6b86 100644
--- a/common/cmd_net.c
+++ b/common/cmd_net.c
@@ -51,7 +51,7 @@
 U_BOOT_CMD(
 	tftpboot,	3,	1,	do_tftpb,
 	"tftpboot- boot image via network using TFTP protocol\n",
-	"[loadAddress] [bootfilename]\n"
+	"[loadAddress] [[hostIPaddr:]bootfilename]\n"
 );
 
 int do_rarpb (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])