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/CarMediaLab/flea3/flea3.c b/board/CarMediaLab/flea3/flea3.c
index 3cd4dc9..5e75eb0 100644
--- a/board/CarMediaLab/flea3/flea3.c
+++ b/board/CarMediaLab/flea3/flea3.c
@@ -211,7 +211,7 @@
 		{ "mxc_nand", MTD_DEV_TYPE_NAND, }, /* NAND flash */
 	};
 
-	if (getenv("fdt_noauto")) {
+	if (env_get("fdt_noauto")) {
 		puts("   Skiping ft_board_setup (fdt_noauto defined)\n");
 		return 0;
 	}