Bring-up KMAC-ROM checking

[elliotb-lowrisc]

Bring-up KMAC by getting it working with the ROM Control scrambled ROM checking feature. No HAL is needed as it is only going to be used with ROM Control via an app interface, so no TL-UL connection will be made to KMAC.

[elliotb-lowrisc]

KMAC and ROM Control are now working together seemingly successfully, but now need a scrambled software image in order to be able to boot. Need to port the "scramble_image.py" script from OT and integrate it.

[elliotb-lowrisc]

• Added axi_isolate to ROM access path enact fetch gating controlled by ROM checker. Confirmed it stalled fetching when ROM check fails.
• Got scramble script working manually. Confirmed SW processed by it was allowed to fetch/boot by ROM checker and ran to program completion.

Next, the scramble script needs to be integrated into the SW build flow.

[elliotb-lowrisc]

I've created a draft PR with the current state of things (i.e. scramble script needing integrating): #590

[elliotb-lowrisc]

New sibling-task created for scrambling script integration and final merge: #591