build: don't regenerate makefiles constantly

The phoney prepare-dir target means we keep regenerating makefiles.  Avoid
it by folding it into the regular target.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: ask before nuking old build/release/

Suggested by Nadav.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>