mprotect() should not fail if it encounters non present pte

mprotect() should fails with ENOMEM if it is called on non mapped
virtual address, but this check is done by mmu::ismapped().

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Revert "build: allow image cacheing during build"

This reverts commit 97626d36eeeda9a59576efeb1da0af488cdb79b2. As pointed
out Tomek, "-u" flag is already passed in mkzfs.py.