kernel: Fix environment pointer setup in ar71xx/ath79

Observed on ar71xx/ath79 platforms such as Ubiquiti RouterStations.Reported in #20642.:
(https://dev.openwrt.org/ticket/20642).

If embedded command-line text exists with CONFIG_IMAGE_CMDLINE_HACK=y,firmware
init doesn't initialize environment pointer (fw_init_cmdline not called).

arcs_cmdline is not initialized before calling strlcat.

Signed-off-by: Ted Hess <thess@kitschensync.net>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>

SVN-Revision: 47270

target/linux/ar71xx/patches-4.1/508-MIPS-ath79-prom-image-command-line-hack.patch

@@ -1,6 +1,6 @@
 --- a/arch/mips/ath79/prom.c
 +++ b/arch/mips/ath79/prom.c
-@@ -33,6 +33,35 @@ static void __init ath79_prom_append_cmd
+@@ -33,6 +33,41 @@ static void __init ath79_prom_append_cmd
  	strlcat(arcs_cmdline, ath79_cmdline_buf, sizeof(arcs_cmdline));
  }
  
@@ -27,6 +27,12 @@
 +		strlcat(arcs_cmdline, p, sizeof(arcs_cmdline));
 +	}
 +
++	/* Validate and setup environment pointer */
++	if (fw_arg2 < CKSEG0)
++		_fw_envp = NULL;
++	else
++		_fw_envp = (int *)fw_arg2;
++
 +	return 1;
 +}
 +#else
@@ -36,7 +42,7 @@
  static int __init ath79_prom_init_myloader(void)
  {
  	struct myloader_info *mylo;
-@@ -61,6 +90,8 @@ static int __init ath79_prom_init_myload
+@@ -61,6 +96,8 @@ static int __init ath79_prom_init_myload
  
  	ath79_prom_append_cmdline("ethaddr", mac_buf);
  
@@ -45,7 +51,7 @@
  	return 1;
  }
  
-@@ -71,7 +102,8 @@ void __init prom_init(void)
+@@ -71,7 +108,8 @@ void __init prom_init(void)
  	if (ath79_prom_init_myloader())
  		return;
  
@@ -55,3 +61,13 @@
  
  	env = fw_getenv("ethaddr");
  	if (env)
+--- a/arch/mips/fw/lib/cmdline.c
++++ b/arch/mips/fw/lib/cmdline.c
+@@ -35,6 +35,7 @@ void __init fw_init_cmdline(void)
+ 	else
+ 		_fw_envp = (int *)fw_arg2;
+ 
++	arcs_cmdline[0] = '\0';
+ 	for (i = 1; i < fw_argc; i++) {
+ 		strlcat(arcs_cmdline, fw_argv(i), COMMAND_LINE_SIZE);
+ 		if (i < (fw_argc - 1))