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/tcl/sl50/board.c b/board/tcl/sl50/board.c
index cc65c7b..08af279 100644
--- a/board/tcl/sl50/board.c
+++ b/board/tcl/sl50/board.c
@@ -322,7 +322,7 @@
 
 #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))
@@ -340,7 +340,7 @@
 	mac_addr[4] = mac_lo & 0xFF;
 	mac_addr[5] = (mac_lo & 0xFF00) >> 8;
 
-	if (!getenv("eth1addr")) {
+	if (!env_get("eth1addr")) {
 		if (is_valid_ethaddr(mac_addr))
 			eth_env_set_enetaddr("eth1addr", mac_addr);
 	}