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>

drivers/acpi/Kconfig

@@ -283,24 +283,23 @@ config ACPI_TOSHIBA
 	  If you have a legacy free Toshiba laptop (such as the Libretto L1
 	  series), say Y.
 
-config ACPI_CUSTOM_DSDT
-	bool "Include Custom DSDT"
+config ACPI_CUSTOM_DSDT_FILE
+	string "Custom DSDT Table file to include"
+	default ""
 	depends on !STANDALONE
-	default n 
 	help
 	  This option supports a custom DSDT by linking it into the kernel.
 	  See Documentation/acpi/dsdt-override.txt
 
-	  If unsure, say N.
-
-config ACPI_CUSTOM_DSDT_FILE
-	string "Custom DSDT Table file to include"
-	depends on ACPI_CUSTOM_DSDT
-	default ""
-	help
 	  Enter the full path name to the file which includes the AmlCode
 	  declaration.
 
+	  If unsure, don't enter a file name.
+
+config ACPI_CUSTOM_DSDT
+	bool
+	default ACPI_CUSTOM_DSDT_FILE != ""
+
 config ACPI_CUSTOM_DSDT_INITRD
 	bool "Read Custom DSDT from initramfs"
 	depends on BLK_DEV_INITRD