Change DRVS file requirements

A DRVS file isn't necessarily needed for operation of the machine, so I
changed it to where the kernel can continue to operate without said
file.

int21.asm

@@ -170,6 +170,10 @@ find_files_loop:
  mov ah, byte ptr es:bx
  cmp ah, 80h
  je found_field
+ ; If there is a null here, then we are in empty spce of the FSB.
+ ; Exit just in case.
+ cmp ah, 00h
+ je disk_error
  inc bx
  jmp find_files_loop
 disk_error:

kernel.asm

@@ -197,7 +197,8 @@ c1:
  mov bx, 0000h
  int 21h
  ;; First check for an error, and if so handle it appropriately
- jc disk_error
+ ;; IF there is no DRVS file, simply skip over this bit
+ jc drv_nofile
  ;; Now we need to parse the data. Formatting is as shown:
  ;; 0x55FF00AA #<filename1>* #<filename2>* ... 0x0D
  ;; This is all in one big string, the CR is the EOF mark (spaces not in file).
@@ -922,5 +923,11 @@ a20_ns:
  int 10h
  jmp halt_forever
 
+drv_nofile:
+ mov ah, 01h
+ mov dx, missing_drvs
+ int 21h
+ ; Skip past driver processing
+ jmp command_prompt
 ; At current test build, we have 1429 bytes.
 times 1536-($-$$) db 00h