targets/i386/NEPETA: fix boot.config; disable DMA to allow the CF card to boot; enable DMA on other disks during boot
The generic direct-insertion CF-IDE adapter I have doesn't properly pass DMA
signals, so PIO modes work perfectly but DMA modes fail with DMA
timeouts. Forbid the kernel from automatically setting DMA modes on any
ATA devices, so that root can be mounted from the CF card; instead, set
DMA modes for capable data drives after boot.