Currently the parameter was read from the generated Makefile which was
not re-generated on incremental build. The fix is to move the default
to build.mk, this way the default will always be picked unless masked
by command line argument.

Fixes #153

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>