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/common/fb_mmc.c b/common/fb_mmc.c
index 2113b6c..5065ad5 100644
--- a/common/fb_mmc.c
+++ b/common/fb_mmc.c
@@ -49,7 +49,7 @@
 		/* check for alias */
 		strcpy(env_alias_name, "fastboot_partition_alias_");
 		strncat(env_alias_name, name, 32);
-		aliased_part_name = getenv(env_alias_name);
+		aliased_part_name = env_get(env_alias_name);
 		if (aliased_part_name != NULL)
 			ret = part_get_info_by_name(dev_desc,
 					aliased_part_name, info);