Add additional config options to remove parts of code.

Added options CONFIG_BOOT, CONFIG_SERIAL, CONFIG_LPT, CONFIG_KEYBOARD,
    CONFIG_BOOTMENU.
Also extended coverage of existing options to ensure full code got
    removed.
diff --git a/src/boot.c b/src/boot.c
index b090058..3efa9b6 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -77,6 +77,9 @@
 static void
 try_boot(u16 seq_nr)
 {
+    if (! CONFIG_BOOT)
+        BX_PANIC("Boot support not compiled in.\n");
+
     SET_EBDA(ipl.sequence, seq_nr);
 
     u32 bootdev = GET_EBDA(ipl.bootorder);