ipkg: fix MMU-less build

ipkg: fix MMU-less build

The code incorrectly tests for __ARCH_HAS_MMU__ which is true for
architecture that *may* have a MMU, instead of __ARCH_USE_MMU__ which
specifies if MMU support is really enabled. Add a patch making direct
use of HAVE_FORK that configure defines when fork() is detected.

Fixes:
http://autobuild.buildroot.net/results/6cf/6cf75e08795d9ab194ce4e882c0f4858bad979c3/
http://autobuild.buildroot.net/results/964/964d8f694bc7d05b35411eabfbadf40bbf6337ae/
http://autobuild.buildroot.net/results/0f2/0f2cddf89af3ad4330556acd04ab6cb080370e24/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>