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/drivers/usb/gadget/f_fastboot.c b/drivers/usb/gadget/f_fastboot.c
index 7cd6d24..d05b74b 100644
--- a/drivers/usb/gadget/f_fastboot.c
+++ b/drivers/usb/gadget/f_fastboot.c
@@ -212,7 +212,7 @@
 		f->hs_descriptors = fb_hs_function;
 	}
 
-	s = getenv("serial#");
+	s = env_get("serial#");
 	if (s)
 		g_dnl_set_serialnumber((char *)s);
 
@@ -426,7 +426,7 @@
 		sprintf(str_num, "0x%08x", CONFIG_FASTBOOT_BUF_SIZE);
 		strncat(response, str_num, chars_left);
 	} else if (!strcmp_l1("serialno", cmd)) {
-		s = getenv("serial#");
+		s = env_get("serial#");
 		if (s)
 			strncat(response, s, chars_left);
 		else
@@ -441,7 +441,7 @@
 		}
 
 		sprintf(envstr, "fastboot.%s", cmd);
-		s = getenv(envstr);
+		s = env_get(envstr);
 		if (s) {
 			strncat(response, s, chars_left);
 		} else {