build: build seastar in build directory
build: build seastar in build directory

Currently, seastar is built in seastar/build/{mode}. This means we have two build
directories: build/{mode} and seastar/build/{mode}.

This patch changes that to have only a single build directory (build/{mode}). It
does that by calling Seastar's cmake directly instead of through Seastar's
./configure.py.  However, to support dpdk, if that is enabled it calls cmake
through Seastar's ./cooking.sh (similar to what Seastar's ./configure.py does).

All ./configure.py flags are translated to cmake variables, in the same way that
Seastar does.

Contains fix from Rafael to pass the flags for the correct mode.
scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla