build: speed up native null build

Drop implicit makefile rules; this prevents make from looking for fortran
source files, etc.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: don't delete loader.img

Removing loader.img causes a null build to rebuild it and usr.img.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Merge branch 'build'

- faster builds

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