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/keymile/km_arm/km_arm.c b/board/keymile/km_arm/km_arm.c
index 08ae9ae..af1ebc4 100644
--- a/board/keymile/km_arm/km_arm.c
+++ b/board/keymile/km_arm/km_arm.c
@@ -201,7 +201,7 @@
 #if defined(CONFIG_KM_MGCOGE3UN)
 	char *wait_for_ne;
 	u8 dip_switch = kw_gpio_get_value(KM_FLASH_ERASE_ENABLE);
-	wait_for_ne = getenv("waitforne");
+	wait_for_ne = env_get("waitforne");
 
 	if ((wait_for_ne != NULL) && (dip_switch == 0)) {
 		if (strcmp(wait_for_ne, "true") == 0) {