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/bosch/shc/board.c b/board/bosch/shc/board.c
index b4c6a0b..999ed95 100644
--- a/board/bosch/shc/board.c
+++ b/board/bosch/shc/board.c
@@ -545,7 +545,7 @@
 
 #if (defined(CONFIG_DRIVER_TI_CPSW) && !defined(CONFIG_SPL_BUILD)) || \
 	(defined(CONFIG_SPL_ETH_SUPPORT) && defined(CONFIG_SPL_BUILD))
-	if (!getenv("ethaddr")) {
+	if (!env_get("ethaddr")) {
 		printf("<ethaddr> not set. Validating first E-fuse MAC\n");
 
 		if (is_valid_ethaddr(mac_addr))