Update to minios-v0.4.2

This adds an explicit linker section for the boot code, meaning we can ensure it always appears at the start of the image without requiring minios.a to be first in the linker command, which in turn allows untangling the linker command generated by `mirage`. Having `_start` at the beginning is obviously needed on ARM (where it is the zImage header), but it also seems to be required on x86 for some reason.

It would be good If some linker guru would check my solution (tested on x86 and ARM and seems to work):

https://github.com/talex5/xen/commit/933978d7a0ce87907f1ce62d199e0fa5884f1975