FS#2764 - ucert: ucert -A output is invalid

[openwrt-bot]

hmh:

ucert -A generates something that ucert cannot read back.

This happens because ucert -A appears to be appending several libubox "blobs" together, but cert_load() uses libubox' blob_parse_untrusted() which expects to get a buffer with a single blob inside, and no padding.

Thus a valid ucert file with certificate and signature will be bigger than the first blob inside it, and blob_parse_untrusted() in libubox won't even try to process it.

Since this renders ucert useless, severity is set to high.

This is related to FS#2762 and FS#2763