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/common/ivm.c b/board/keymile/common/ivm.c
index dc40a83..f1321d9 100644
--- a/board/keymile/common/ivm.c
+++ b/board/keymile/common/ivm.c
@@ -261,7 +261,7 @@
 
 	GET_STRING("IVM_Symbol", IVM_POS_SYMBOL_ONLY, 8)
 	GET_STRING("IVM_DeviceName", IVM_POS_SHORT_TEXT, 64)
-	tmp = (unsigned char *) getenv("IVM_DeviceName");
+	tmp = (unsigned char *)env_get("IVM_DeviceName");
 	if (tmp) {
 		int	len = strlen((char *)tmp);
 		int	i = 0;