Rework keyring check to support busybox cmp

Apertis uses busybox cmp which lacks of long options in general and does not
support --bytes. Under this circumstances rework its use to avoid these
limitations.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

cmdline/apt-key.in

@@ -325,16 +325,20 @@ is_supported_keyring() {
     local FILEEXT="${1##*.}"
     if [ "$FILEEXT" = 'gpg' ]; then
 	# 0x98, 0x99 and 0xC6 via octal as hex isn't supported by dashs printf
-	if printf '\231' | cmp --silent --bytes=1 - "$1"; then
+	local BYTE_TMP_FILE=$(mktemp)
+	head -c 1 "$1" > "$BYTE_TMP_FILE"
+	if printf '\231' | cmp -s - "$BYTE_TMP_FILE"; then
 	    true
-	elif printf '\230' | cmp --silent --bytes=1 - "$1"; then
+	elif printf '\230' | cmp -s - "$BYTE_TMP_FILE"; then
 	    true
-	elif printf '\306' | cmp --silent --bytes=1 - "$1"; then
+	elif printf '\306' | cmp -s - "$BYTE_TMP_FILE"; then
 	    true
 	else
 	    apt_warn "The key(s) in the keyring $1 are ignored as the file has an unsupported filetype."
+	    rm "$BYTE_TMP_FILE"
 	    return 1
 	fi
+	rm "$BYTE_TMP_FILE"
     elif [ "$FILEEXT" = 'asc' ]; then
 	true #dearmor_filename will deal with them
     else