Each of the submodule configuration files defined a subset of the
cross compiler tools that it used and many were picking up the
system `ar`, `nm`, `strip, `ld`, etc. They all now use a `Makefile`
macro that defines the path to the proper cross compiler tools.
For ones that need the tools, but not the musl-libc gcc,
there is $(CROSS_TOOLS_NOCC) that is all of them without gcc.
This is for musl-libc itself, as well as xen and the Linux kernel.
This addresses multiple issues:
* Issue #63: initrd is build fresh each time, so tracked files do not matter.
* Issue #144: build time configuration
* Issue #123: allows us to customize the startup experience
* Issue #122: manual start-xen will go away
* Issue #25: tpmtotp PCRs are updated after reading the secret
* Issue #16: insmod now meaures modules
