package/pkg-utils: escape \ in generated legal-info

package/pkg-utils: escape \ in generated legal-info

In the output of legal-info, which is JSON-formatted, we include the
CPI_ID (when it is valid).

For xerces, the CPE_ID contains two sequences of \+ (which is exactly
what is present in the NIST DB, [0]).

However, in JSON, like in C, \ escapes the following character; only a
very limited set of characters are valid to escape: " \ / b f n r t u.
Escaping any other character is invalid. Conformant JSON parser will
choke on invalid sequences, and so does not the json python module:

      File "/usr/lib/python2.7/json/decoder.py", line 380, in raw_decode
        obj, end = self.scan_once(s, idx)
    ValueError: Invalid \escape: line 1 column 608554 (char 608553)

We fix that be globally escaping \ in our json output, in the generic
sanitising macro.

[0] https://nvd.nist.gov/products/cpe/detail/645?namingFormat=2.3&orderBy=CPEURI&keyword=xerces&status=FINAL

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>