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/net/tftp.c b/net/tftp.c
index 61e1671..a5ed8c5 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -706,11 +706,11 @@
 	 * TFTP protocol has a minimal timeout of 1 second.
 	 */
 
-	ep = getenv("tftpblocksize");
+	ep = env_get("tftpblocksize");
 	if (ep != NULL)
 		tftp_block_size_option = simple_strtol(ep, NULL, 10);
 
-	ep = getenv("tftptimeout");
+	ep = env_get("tftptimeout");
 	if (ep != NULL)
 		timeout_ms = simple_strtol(ep, NULL, 10);
 
@@ -720,7 +720,7 @@
 		timeout_ms = 1000;
 	}
 
-	ep = getenv("tftptimeoutcountmax");
+	ep = env_get("tftptimeoutcountmax");
 	if (ep != NULL)
 		tftp_timeout_count_max = simple_strtol(ep, NULL, 10);
 
@@ -822,10 +822,10 @@
 	tftp_our_port = 1024 + (get_timer(0) % 3072);
 
 #ifdef CONFIG_TFTP_PORT
-	ep = getenv("tftpdstp");
+	ep = env_get("tftpdstp");
 	if (ep != NULL)
 		tftp_remote_port = simple_strtol(ep, NULL, 10);
-	ep = getenv("tftpsrcp");
+	ep = env_get("tftpsrcp");
 	if (ep != NULL)
 		tftp_our_port = simple_strtol(ep, NULL, 10);
 #endif