mb/x/acpi_tables: Move EC_RW detection

These boards without ChromeEC do not set ACTIVE_EC_RW
flag as part of the gnvs_assign_chromeos() function.
Create abstraction to avoid <vendorcode/chromeos/x> include.

Change-Id: Ic6029e1807fcfe7dd2c766ce8221e347b6b096f9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48777
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/acpi/acpi_gnvs.h

@@ -10,6 +10,7 @@ void *gnvs_get_or_create(void);
 void acpi_inject_nvsa(void);
 
 void gnvs_assign_chromeos(void);
+void gnvs_set_ecfw_rw(void);
 
 /* Platform code must implement these. */
 struct global_nvs;