add static, hand-written <ukernel/config.h>

It has a single flag: whether the x86 SYSENTER/SYSEXIT mechanism should be
supported. The negative case hasn't been tested, but the positive works.
generate dependency information for assembly sources

Because it's possible, and because they'll have those also.
set %eip in softint syscall & exregs ISR bottom halves

The caller expects this because it may return via the scheduler.
ipc: set passive sender's registers in ipc_recv_half()

Oops.