acpi: rework BERT ACPI table generation logic

Check if the ACPI_BERT Kconfig option is selected and only then try to
generate the BERT table. Also remove the acpi_is_boot_error_src_present
weak function from the ACPI global compilation unit and use the return
value of acpi_soc_get_bert_region to determine if there is a valid BERT
region with logged errors.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I2a281f5f636010ba3b2e7e097e9cf53683022aea
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55054
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/include/acpi/acpi.h

@@ -1341,7 +1341,6 @@ void acpi_create_lpit(acpi_lpit_t *lpit);
 unsigned long acpi_create_lpi_desc_ncst(acpi_lpi_desc_ncst_t *lpi_desc, uint16_t uid);
 
 /* For crashlog. */
-bool acpi_is_boot_error_src_present(void);
 enum cb_err acpi_soc_get_bert_region(void **region, size_t *length);
 
 /* For ACPI S3 support. */