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/nokia/rx51/rx51.c b/board/nokia/rx51/rx51.c
index 889e748..7764288 100644
--- a/board/nokia/rx51/rx51.c
+++ b/board/nokia/rx51/rx51.c
@@ -234,18 +234,18 @@
 	params->u.core.rootdev = 0x0;
 
 	/* append omap atag only if env setup_omap_atag is set to 1 */
-	str = getenv("setup_omap_atag");
+	str = env_get("setup_omap_atag");
 	if (!str || str[0] != '1')
 		return;
 
-	str = getenv("setup_console_atag");
+	str = env_get("setup_console_atag");
 	if (str && str[0] == '1')
 		setup_console_atag = 1;
 	else
 		setup_console_atag = 0;
 
-	setup_boot_reason_atag = getenv("setup_boot_reason_atag");
-	setup_boot_mode_atag = getenv("setup_boot_mode_atag");
+	setup_boot_reason_atag = env_get("setup_boot_reason_atag");
+	setup_boot_mode_atag = env_get("setup_boot_mode_atag");
 
 	params = *in_params;
 	t = (struct tag_omap *)&params->u;