env: Rename getenv/_f() to env_get()

We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/common/sys_eeprom.c b/board/freescale/common/sys_eeprom.c
index c593cff..152ad84 100644
--- a/board/freescale/common/sys_eeprom.c
+++ b/board/freescale/common/sys_eeprom.c
@@ -478,7 +478,7 @@
 			/* Only initialize environment variables that are blank
 			 * (i.e. have not yet been set)
 			 */
-			if (!getenv(enetvar))
+			if (!env_get(enetvar))
 				env_set(enetvar, ethaddr);
 		}
 	}