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>
Revert "mempool: use new page range allocator"

This reverts commit 9202b754911e59f1c958d1be5d322edad4a4a9ee; causes assert
failures with iperf:

Backtrace:
    at /home/vladz/work/osv/external/x64/gcc.bin/usr/include/c++/4.8.2/bits/vector.tcc:519
    at /home/vladz/work/osv/external/x64/gcc.bin/usr/include/c++/4.8.2/bits/stl_vector.h:1024

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