env: Rename getenv_hex(), getenv_yesno(), getenv_ulong()

We are now using an env_ prefix for environment functions. Rename these
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/fdt.c

@@ -111,7 +111,7 @@ static int do_fdt(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 				return 1;
 			printf("The address of the fdt is %#08lx\n",
 			       control ? (ulong)map_to_sysmem(blob) :
-					getenv_hex("fdtaddr", 0));
+					env_get_hex("fdtaddr", 0));
 			return 0;
 		}