lib.sh: hide stderr on download()

on the initial check, the output is confusing because
it will say "checksum verification failed" if the
file doesn't already exist, but then goes to download.

only say checksum failed if a download occured, and the
check failed, otherwise report nothing except that the
file already exists.

this will not reduce the ability to debug issues later
on, and it will reduce the amount of confusion for users.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -230,7 +230,7 @@ singletree()
 download()
 {
 	dl_fail="y" # 1 url, 2 url backup, 3 destination, 4 checksum
-	vendor_checksum "$4" "$3" || dl_fail="n"
+	vendor_checksum "$4" "$3" 2>/dev/null || dl_fail="n"
 	[ "$dl_fail" = "n" ] && e "$3" f && return 0
 	x_ mkdir -p "${3%/*}" && for url in "$1" "$2"; do
 		[ "$dl_fail" = "n" ] && break