build: allow image cacheing during build

Since there's no user data on the image, we can safely cache it, improving
build performance.  Helpful duing 'git bisect'.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
runtime: Fix hard-coded _SC_PAGESIZE

Just came across this. Trivial to fix.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>