msr: Add new MSR modules (rdmsr/wrmsr)

In order to be able to read from and write to model-specific registers,
two new modules are added. They are i386 specific, as the cpuid module.

rdmsr module registers the command rdmsr that allows reading from a MSR.
wrmsr module registers the command wrmsr that allows writing to a MSR.

wrmsr module is disabled if UEFI secure boot is enabled.

Please note that on SMP systems, interacting with a MSR that has a scope
per hardware thread, implies that the value only applies to the
particular cpu/core/thread that ran the command.

Also, if you specify a reserved or unimplemented MSR address, it will
cause a general protection exception (which is not currently being
handled) and the system will reboot.

Signed-off-by: Jesús Diéguez Fernández <jesusdf@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/efi/shim_lock.c

@@ -43,7 +43,7 @@ static grub_efi_guid_t shim_lock_guid = GRUB_EFI_SHIM_LOCK_GUID;
 static grub_efi_shim_lock_protocol_t *sl;
 
 /* List of modules which cannot be loaded if UEFI secure boot mode is enabled. */
-static const char * const disabled_mods[] = {"iorw", "memrw", NULL};
+static const char * const disabled_mods[] = {"iorw", "memrw", "wrmsr", NULL};
 
 static grub_err_t
 shim_lock_init (grub_file_t io, enum grub_file_type type,