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/common/image-android.c b/common/image-android.c
index c668407..018c759 100644
--- a/common/image-android.c
+++ b/common/image-android.c
@@ -90,7 +90,7 @@
 	if (*hdr->cmdline)
 		strcat(newbootargs, hdr->cmdline);
 
-	setenv("bootargs", newbootargs);
+	env_set("bootargs", newbootargs);
 
 	if (os_data) {
 		*os_data = (ulong)hdr;