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/siemens/common/board.c b/board/siemens/common/board.c
index b967227..65fa6af 100644
--- a/board/siemens/common/board.c
+++ b/board/siemens/common/board.c
@@ -121,7 +121,7 @@
 	char *ptr_env;
 
 	/* If button is not found we take default */
-	ptr_env = getenv(envname);
+	ptr_env = env_get(envname);
 	if (NULL == ptr_env) {
 		gpio = def;
 	} else {
@@ -199,7 +199,7 @@
 		strcat(str_tmp, num);
 
 		/* If env var is not found we stop */
-		ptr_env = getenv(str_tmp);
+		ptr_env = env_get(str_tmp);
 		if (NULL == ptr_env)
 			break;