coreboot

Date back to coreboot 4.9, mainboard_vbt_filename() in src/mainboard/lenovo/x200/blc.c calls get_blc_pwm_freq_value(NULL), making the following if condition always falling to the first, not to mention null dereference on strcmp() in get_blc_pwm_freq_value().

In order to determine the correct vbt filename at runtime, edid ascii string should be obtained first.

This issue may be distantly related to #474.