ACPI: prevent randconfig build failure on empty ACPI_CUSTOM_DSDT_FILE

Make ACPI_CUSTOM_DSDT boolean config symbol a hidden and derived
value, based on the value of ACPI_CUSTOM_DSDT_FILE (string).
Only the latter is presented to the user as a config option.

This fixes problems with "make randconfig" setting ACPI_CUSTOM_DSDT
but leaving ACPI_CUSTOM_DSDT_FILE empty/blank.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Len Brown <len.brown@intel.com>