refactor(generate_json): use multiline strings in `generate_json.sh`

Using multiline strings (delimited by `EOF` on each side) means the
contents of the json file can be inserted verbatim into
`generate_json.sh`, rather than inserting escape codes (`\t` and `\n`)
manually.

Signed-off-by: Karl Meakin <karl.meakin@arm.com>
Change-Id: I85b8287712ea4cb99e70c58482df0d886b780dde
2 files changed