Set MBR bootloader code to busy loop on x86

Booting from one of our images on a PC could be seriously harmful. This
patch adds a few standard instructions to the MBR bootloader code to
hang a booting PC if someone tries to boot from our image.

Change-Id: I85af964595c7c63cbb5e456db87d30cfdb6ddadc
Test: Updated unit tests pass
Bug: None