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/st/stm32f429-discovery/stm32f429-discovery.c b/board/st/stm32f429-discovery/stm32f429-discovery.c
index 673e772..d6763c3 100644
--- a/board/st/stm32f429-discovery/stm32f429-discovery.c
+++ b/board/st/stm32f429-discovery/stm32f429-discovery.c
@@ -314,7 +314,7 @@
 	char serialno[25];
 	uint32_t u_id_low, u_id_mid, u_id_high;
 
-	if (!getenv("serial#")) {
+	if (!env_get("serial#")) {
 		u_id_low  = readl(&STM32_U_ID->u_id_low);
 		u_id_mid  = readl(&STM32_U_ID->u_id_mid);
 		u_id_high = readl(&STM32_U_ID->u_id_high);