[roll] Roll fuchsia [kernel][phys] Add physboot version loading an ELF kernel

This adds a second version of physboot that's closer to its final
form.  It loads an ELF (PIE) kernel image and hands off to its
entry point directly, rather than loading a ZBI kernel and using
the ZBI boot protocol.  The kernel is relocated for its final
virtual address (currently stubbed out to a default virtual
address), but still entered in physical address mode.  The ELF
kernel image is found in the kernel package as "physzircon".

This also adds the `kernel.select` boot option to implement
"kernel selection": choosing the kernel package to fetch out of
the STORAGE_KERNEL filesystem.

This is all tested with a trivial stub kernel whose code is
sufficiently purely position-independent that it doesn't care
that its relocation (virtual) address and runtime (physical)
address don't match.  It simply verifies that code-patching has
happened, and then prints the boot-test success string.

Original-Bug: 91187
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/810287
Original-Revision: d83ccdfd3fd790fe71023dd0772afecdab4585c7
GitOrigin-RevId: 2562055dd74173b0bc1db9521633179e7b93c6ba
Change-Id: I54706ea20804db8412cc3d1d8f57be0731e652cf