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/xtensa/lib/bootm.c b/arch/xtensa/lib/bootm.c
index 1604bb9..16961ac 100644
--- a/arch/xtensa/lib/bootm.c
+++ b/arch/xtensa/lib/bootm.c
@@ -136,7 +136,7 @@
 {
 	struct bp_tag *params, *params_start;
 	ulong initrd_start, initrd_end;
-	char *commandline = getenv("bootargs");
+	char *commandline = env_get("bootargs");
 
 	if (!(flag & (BOOTM_STATE_OS_GO | BOOTM_STATE_OS_FAKE_GO)))
 		return 0;