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/xes/common/board.c b/board/xes/common/board.c
index 4ed6f50..b76eb94 100644
--- a/board/xes/common/board.c
+++ b/board/xes/common/board.c
@@ -51,13 +51,13 @@
 
 	/* Display board specific information */
 	puts("       ");
-	i = getenv_f("board_rev", buf, sizeof(buf));
+	i = env_get_f("board_rev", buf, sizeof(buf));
 	if (i > 0)
 		printf("Rev %s, ", buf);
-	i = getenv_f("serial#", buf, sizeof(buf));
+	i = env_get_f("serial#", buf, sizeof(buf));
 	if (i > 0)
 		printf("Serial# %s, ", buf);
-	i = getenv_f("board_cfg", buf, sizeof(buf));
+	i = env_get_f("board_cfg", buf, sizeof(buf));
 	if (i > 0)
 		printf("Cfg %s", buf);
 	puts("\n");