Overhaul option rom processing.

Add initial support for scanning PCI devices for option roms.
Implement two pass option rom scan - init first then scan for bev/bcv.
Support calling BCV vectors that are found.
diff --git a/src/post.c b/src/post.c
index 1e9e818..619082b 100644
--- a/src/post.c
+++ b/src/post.c
@@ -222,6 +222,9 @@
     timer_setup();
     mathcp_setup();
 
+    memmap_setup();
+    ram_probe();
+
     vga_setup();
 
     kbd_setup();
@@ -229,8 +232,6 @@
     serial_setup();
     mouse_setup();
 
-    memmap_setup();
-    ram_probe();
     pci_bios_setup();
     init_bios_tables();
     memmap_finalize();