[sw, MaskROM] Minor `mask_rom_start.S` improvements.
[sw, MaskROM] Minor `mask_rom_start.S` improvements.

- `mstatus` `mie` bit is cleared on reset.
  Volume II: RISC-V Privileged Architectures V20190608-Priv-MSU-Ratified:
  "3.3 Reset
  Upon reset, a hart’s privilege mode is set to M. The mstatus fields MIE and MPRV are reset to 0."

- Reference to `boot.md` has been added in a comment.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

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