Add a new config_HsumBootStrategy to control the boot strategy in Headless System User Mode (HSUM)

This config will be used to control the boot strategy in HSUM.
This CL adds the config. A follow-up CL will actually implement the logic.

Context: see DD in the bug

Bug: 374926694
Change-Id: I460e5151cf47d565bed30f2aa6eb1ed9df5e2336
Flag: EXEMPT adding new config only
Test: manually build and deploy in DUT