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/arch/arm/mach-tegra/board2.c b/arch/arm/mach-tegra/board2.c
index bd13796..0426b7a 100644
--- a/arch/arm/mach-tegra/board2.c
+++ b/arch/arm/mach-tegra/board2.c
@@ -214,9 +214,9 @@
 #if defined(CONFIG_TEGRA_SUPPORT_NON_SECURE)
 	if (tegra_cpu_is_non_secure()) {
 		printf("CPU is in NS mode\n");
-		setenv("cpu_ns_mode", "1");
+		env_set("cpu_ns_mode", "1");
 	} else {
-		setenv("cpu_ns_mode", "");
+		env_set("cpu_ns_mode", "");
 	}
 #endif
 	start_cpu_fan();