cmd: nvedit: Mark a few functions static

Some functions are not used anywhere except the same file
where they are defined. Mark them static. This helps avoiding
the compiler warnings:

  cmd/nvedit.c:201:5: warning: no previous prototype for ‘do_env_ask’ [-Wmissing-prototypes]
  cmd/nvedit.c:315:5: warning: no previous prototype for ‘do_env_callback’ [-Wmissing-prototypes]
  cmd/nvedit.c:384:5: warning: no previous prototype for ‘do_env_flags’ [-Wmissing-prototype ]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/nvedit.c b/cmd/nvedit.c
index 74ff5c6..392f90f 100644
--- a/cmd/nvedit.c
+++ b/cmd/nvedit.c
@@ -198,7 +198,7 @@
  * Prompt for environment variable
  */
 #if defined(CONFIG_CMD_ASKENV)
-int do_env_ask(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
+static int do_env_ask(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
 	char message[CONFIG_SYS_CBSIZE];
 	int i, len, pos, size;
@@ -312,8 +312,8 @@
 /*
  * Print the callbacks available and what they are bound to
  */
-int do_env_callback(struct cmd_tbl *cmdtp, int flag, int argc,
-		    char *const argv[])
+static int do_env_callback(struct cmd_tbl *cmdtp, int flag, int argc,
+			   char *const argv[])
 {
 	struct env_clbk_tbl *clbkp;
 	int i;
@@ -381,7 +381,7 @@
 /*
  * Print the flags available and what variables have flags
  */
-int do_env_flags(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
+static int do_env_flags(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
 	/* Print the available variable types */
 	printf("Available variable type flags (position %d):\n",