efi: Make it possible to disable efivar_ssdt entirely

efi: Make it possible to disable efivar_ssdt entirely

In most cases, such as CONFIG_ACPI_CUSTOM_DSDT and
CONFIG_ACPI_TABLE_UPGRADE, boot-time modifications to firmware tables
are tied to specific Kconfig options.  Currently this is not the case
for modifying the ACPI SSDT via the efivar_ssdt kernel command line
option and associated EFI variable.

This patch adds CONFIG_EFI_CUSTOM_SSDT_OVERLAYS, which defaults
disabled, in order to allow enabling or disabling that feature during
the build.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Jones <pjones@redhat.com>
Link: https://lore.kernel.org/r/20200615202408.2242614-1-pjones@redhat.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>