json: Implement wrapping interface

json: Implement wrapping interface

While the newly added jsmn library provides the parsing interface, it
does not provide any kind of interface to act on parsed tokens. Instead,
the caller is expected to handle pointer arithmetics inside of the token
array in order to extract required information. While simple, this
requires users to know some of the inner workings of the library and is
thus quite an unintuitive interface.

This commit adds a new interface on top of the jsmn parser that provides
convenience functions to retrieve values from the parsed json type, grub_json_t.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>