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/cmd/bootm.c b/cmd/bootm.c
index daf15d9..df0bbe1 100644
--- a/cmd/bootm.c
+++ b/cmd/bootm.c
@@ -138,7 +138,7 @@
 
 int bootm_maybe_autostart(cmd_tbl_t *cmdtp, const char *cmd)
 {
-	const char *ep = getenv("autostart");
+	const char *ep = env_get("autostart");
 
 	if (ep && !strcmp(ep, "yes")) {
 		char *local_args[2];
@@ -202,7 +202,7 @@
 #if defined(CONFIG_CMD_BOOTD)
 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);
 }
 
 U_BOOT_CMD(