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>

arch/arm/cpu/armv7/vf610/generic.c

@@ -329,7 +329,7 @@ int arch_misc_init(void)
 
 	strcpy(soc, "vf");
 	strcat(soc, soc_type);
-	setenv("soc", soc);
+	env_set("soc", soc);
 
 	return 0;
 }