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/gdsys/p1022/controlcenterd.c b/board/gdsys/p1022/controlcenterd.c
index 01064dc..24e58cb 100644
--- a/board/gdsys/p1022/controlcenterd.c
+++ b/board/gdsys/p1022/controlcenterd.c
@@ -223,7 +223,7 @@
 #ifdef CONFIG_TRAILBLAZER
 int do_bootd(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 {
-	return run_command(getenv("bootcmd"), flag);
+	return run_command(env_get("bootcmd"), flag);
 }
 
 int board_early_init_r(void)