ddb: constify command tables

ok jca@

ddb: simplify machine command handling

Define a consistently named db_machine_command_table[] across all
archs that implement the MD "machine" command, and hook this into
the main command table instead of patching it at runtime.

ok mpi@ jca@

Include <sys/mutex.h> to avoid a hidden header dependency.

OK jsg@ deraadt@

Make switching CPUs in DDB work.

return early if either initrd property is missing

avoids unlikely path with use of uninitialised pointer
found with scan-build, ok kettenis@

Enter ddb at a trap instruction or with db_enter().

Tell make that trap_subr.S needs assym.h.
Fill in opal_cngetc() so I can type at the "ddb>" prompt.
Add enough code to trap_subr.S to "c" continue from ddb.
Give registers to ddb.
Try to load symbols from initrd.

ok kettenis@

ddb: simplify machine command handling

Define a consistently named db_machine_command_table[] across all
archs that implement the MD "machine" command, and hook this into
the main command table instead of patching it at runtime.

ok mpi@ jca@

Include <sys/mutex.h> to avoid a hidden header dependency.

OK jsg@ deraadt@

Make switching CPUs in DDB work.

return early if either initrd property is missing

avoids unlikely path with use of uninitialised pointer
found with scan-build, ok kettenis@

Enter ddb at a trap instruction or with db_enter().

Tell make that trap_subr.S needs assym.h.
Fill in opal_cngetc() so I can type at the "ddb>" prompt.
Add enough code to trap_subr.S to "c" continue from ddb.
Give registers to ddb.
Try to load symbols from initrd.

ok kettenis@