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-omap2/boot-common.c b/arch/arm/mach-omap2/boot-common.c
index 2bcc8e9..26245aa 100644
--- a/arch/arm/mach-omap2/boot-common.c
+++ b/arch/arm/mach-omap2/boot-common.c
@@ -241,7 +241,7 @@
 int fb_set_reboot_flag(void)
 {
 	printf("Setting reboot to fastboot flag ...\n");
-	setenv("dofastboot", "1");
+	env_set("dofastboot", "1");
 	env_save();
 	return 0;
 }