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/spl/Kconfig b/common/spl/Kconfig
index 5176857..3c887b3 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -279,7 +279,7 @@
 	  configuration. But some boards use this to support 'Falcon' boot
 	  on EXT2 and FAT, where SPL boots directly into Linux without
 	  starting U-Boot first. Enabling this option will make getenv()
-	  and setenv() available in SPL.
+	  and env_set() available in SPL.
 
 config SPL_SAVEENV
 	bool "Support save environment"