contrib: fix BUILDDIR in gen-bitcoin-conf script and gen-manpages.py

the cmake build steps suggest a build/ directory, which breaks these
scripts. Additionally, in-tree builds are no longer allowed, so it makes
sense to update the code and the README accordingly.

contrib/devtools/gen-bitcoin-conf.sh

@@ -5,7 +5,7 @@
 
 export LC_ALL=C
 TOPDIR=${TOPDIR:-$(git rev-parse --show-toplevel)}
-BUILDDIR=${BUILDDIR:-$TOPDIR}
+BUILDDIR=${BUILDDIR:-$TOPDIR/build}
 BINDIR=${BINDIR:-$BUILDDIR/src}
 BITCOIND=${BITCOIND:-$BINDIR/bitcoind}
 SHARE_EXAMPLES_DIR=${SHARE_EXAMPLES_DIR:-$TOPDIR/share/examples}