curl: don't enable debug builds

In oe-core 27824261 --enable-debug was added to the configure arguments
to turn on debugging symbols.  However, enabling debug mode does more
than turn on debugging symbols and introduces some codepaths that can be
controlled with environment variables.  Bluntly, the curl maintainer
says that --enable-debug should not be used in production:

https://curl.se/mail/lib-2023-01/0039.html



I did a build and verified that the curl-dbg package doesn't massively
shrink, so the debug symbols are still being built.

Remove the debug options and hide them behind a PACKAGECONFIG, with a
comment that it should not be used in production.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>