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/amlogic/odroid-c2/odroid-c2.c b/board/amlogic/odroid-c2/odroid-c2.c
index ca9c13b..611155d 100644
--- a/board/amlogic/odroid-c2/odroid-c2.c
+++ b/board/amlogic/odroid-c2/odroid-c2.c
@@ -51,7 +51,7 @@
 			eth_env_set_enetaddr("ethaddr", mac_addr);
 	}
 
-	if (!getenv("serial#")) {
+	if (!env_get("serial#")) {
 		len = meson_sm_read_efuse(EFUSE_SN_OFFSET, serial,
 			EFUSE_SN_SIZE);
 		if (len == EFUSE_SN_SIZE)