configs: Remove check for kernel in fs.py.

It is *not* true that a kernel is required in FS mode. For example,
in SPARC, gem5 is set up to run actual system firmware which will load
a kernel from the disk image. Other systems can run in a bare metal
mode where they also have no kernel.

If a configuration requires a kernel, it should check for it in C++
where there context lives, not globally in fs.py.

Change-Id: Ib094c29474c248f866bd08d4f975648a2c707a19
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24284
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/example/fs.py

@@ -130,9 +130,6 @@ def build_test_system(np):
 
     if options.kernel is not None:
         test_sys.kernel = binary(options.kernel)
-    else:
-        print("Error: a kernel must be provided to run in full system mode")
-        sys.exit(1)
 
     if options.script is not None:
         test_sys.readfile = options.script
@@ -281,9 +278,6 @@ def build_drive_system(np):
     drive_sys.cpu.connectAllPorts(drive_sys.membus)
     if options.kernel is not None:
         drive_sys.kernel = binary(options.kernel)
-    else:
-        print("Error: a kernel must be provided to run in full system mode")
-        sys.exit(1)
 
     if ObjectList.is_kvm_cpu(DriveCPUClass):
         drive_sys.kvm_vm = KvmVM()