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/common/image-android.c b/common/image-android.c
index 018c759..e74d0aa 100644
--- a/common/image-android.c
+++ b/common/image-android.c
@@ -72,7 +72,7 @@
 		len += strlen(hdr->cmdline);
 	}
 
-	char *bootargs = getenv("bootargs");
+	char *bootargs = env_get("bootargs");
 	if (bootargs)
 		len += strlen(bootargs);