i386, x86_64, ppc: fix switch fallthrough cases with GCC7
In util/getroot and efidisk slightly modify exitsing comment to mostly
retain it but still make GCC7 compliant with respect to fall through
annotation.
In grub-core/lib/xzembed/xz_dec_lzma2.c it adds same comments as
upstream.
In grub-core/tests/setjmp_tets.c declare functions as "noreturn" to
suppress GCC7 warning.
In grub-core/gnulib/regexec.c use new __attribute__, because existing
annotation is not recognized by GCC7 parser (which requires that comment
immediately precedes case statement).
iPXE adds Simple File System Protocol to loaded image handle, as side
effect it also adds Block IO protocol (according to comments, to work
around some bugs in EDK2). GRUB assumes that every device with Block IO
is disk and skips network initialization entirely. But iPXE Block IO
implementation is just a stub which always fails for every operation
so cannot be used. Attempt to detect and skip such devices.
We are using media ID which iPXE sets to "iPXE" and block IO size in
hope that no real device would announce 1B block ...
