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/arch/arm/mach-tegra/tegra186/nvtboot_board.c b/arch/arm/mach-tegra/tegra186/nvtboot_board.c
index 9cd3fe2..b94eb42 100644
--- a/arch/arm/mach-tegra/tegra186/nvtboot_board.c
+++ b/arch/arm/mach-tegra/tegra186/nvtboot_board.c
@@ -35,7 +35,7 @@
 	const u32 *prop;
 
 	/* Already a valid address in the environment? If so, keep it */
-	if (getenv("ethaddr"))
+	if (env_get("ethaddr"))
 		return 0;
 
 	node = fdt_path_offset(nvtboot_blob, "/chosen");