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/samsung/common/exynos5-dt.c b/board/samsung/common/exynos5-dt.c
index 44f412d..ae2a6e6 100644
--- a/board/samsung/common/exynos5-dt.c
+++ b/board/samsung/common/exynos5-dt.c
@@ -179,7 +179,7 @@
 	if (board_is_odroidxu4())
 		return info;
 
-	return getenv("dfu_alt_system");
+	return env_get("dfu_alt_system");
 }
 
 char *get_dfu_alt_boot(char *interface, char *devstr)