env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/pxe.c b/cmd/pxe.c
index 0a07f14..357b969 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -591,7 +591,7 @@
 		char bootargs[CONFIG_SYS_CBSIZE];
 
 		cli_simple_process_macros(label->append, bootargs);
-		setenv("bootargs", bootargs);
+		env_set("bootargs", bootargs);
 	}
 
 	debug("running: %s\n", localcmd);
@@ -695,7 +695,7 @@
 		strcat(bootargs, mac_str);
 
 		cli_simple_process_macros(bootargs, finalbootargs);
-		setenv("bootargs", finalbootargs);
+		env_set("bootargs", finalbootargs);
 		printf("append: %s\n", finalbootargs);
 	}
 
@@ -1674,7 +1674,7 @@
 		filename = getenv("bootfile");
 	else {
 		filename = argv[5];
-		setenv("bootfile", filename);
+		env_set("bootfile", filename);
 	}
 
 	if (strstr(argv[3], "ext2"))