Optionrom code cleanup.

Nearly all calers of callrom() use OPTION_ROM_INITVECTOR - make it
Prefer (void*) to explicit casts.
Rename init_optionrom to init_pcirom; rename verifysize_optionrom to
Add 'isvga' parameter to init_optionrom/init_pcirom; unify vga call
    path to use these functions.
Extract cbfs directory scanning into new function run_cbfs_roms().
Allow roms in "vgaroms/" to run even if no pci vga device exists.
Comment improvements.
diff --git a/src/boot.c b/src/boot.c
index 26427ba..b1dffef 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -379,7 +379,7 @@
     call_boot_entry(bootseg, bootip, bootdrv);
-// Boot from a CD-ROM
+// Boot from a CBFS payload
 static void
 boot_cbfs(struct ipl_entry_s *ie)