curl: add debug info

Currently, curl (and libcurl) is built without debug info, making the
curl-dbg package rather useless. Since debug symbols are automatically
stripped and put in that package by the build system, making sure that
curl is built with -g shouldn't hurt anything, but will help those
that try to debug a libcurl-using application and hence explicitly
include curl-dbg in their rootfs.

Unfortunately, setting --enable-debug then changes the default value
of the optimize option from (assume yes) to (assume no), while also
changing the default value of the curldebug option [which is a
separate thing that actually changes generated code to add some memory
tracking] from (assume no) to (assume yes). So explicitly pass the
appropriate options that make those two have the same value as they
used to have by default.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>