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/board_r.c b/common/board_r.c
index 985aa95..a4a7384 100644
--- a/common/board_r.c
+++ b/common/board_r.c
@@ -658,7 +658,7 @@
 	pram += (LOGBUFF_LEN + LOGBUFF_OVERHEAD) / 1024;
 # endif
 	sprintf(memsz, "%ldk", (long int) ((gd->ram_size / 1024) - pram));
-	setenv("mem", memsz);
+	env_set("mem", memsz);
 
 	return 0;
 }