Complete the kernel bootstrap by adding kexec of Linux from Fiwix.

A tiny bootloader bootstrap has been added to compile the builder-hex0 kernel from hex0 source.
The boot compiler is builder-hex0-x86-stage1.hex0 and builder-hex0-x86-stage1.bin.
The builder-hex0 kernel is now named builder-hex0-x86-stage2.hex0.
The inclusion of a binary seed resolves the problem with the previous strategy which used an
architecture-specific hex0 compiler.

If sysb detects a full disk (i.e. DISK=sda) it now partitions the disk unconditionally because
previously fdisk was reporting existing but empty partitions which resulted in no
parititions being created.

e2fsprogs is now built with --disable-tls because musl was built on Fiwix without full threading
support and mkfs.ext4 was crashing without disabling thread local storage.

kexec-linux writes the linux kernel and initramfs to a RAM drive on Fiwix which ensure
a pre-allocated contiguous memory block. The following is written to the ram drive:
a 32-bit number which is the size of the kernel in bytes, a 32-bit number which is the size
of the initramfs in bytes, followed by the Linux kernel image, followed by the initramfs.

kexec-fiwix invokes a sync syscall to ensure all writes are flushed to
the ram drive and then initiates the kexec by shutting down Fiwix with a reboot syscall.
Fiwix knows whether and how to perform the kexec based on kernel parameters passed to it.

sysa.py

@@ -193,12 +193,24 @@ class SysA(SysGeneral):
 
     def create_builder_hex0_disk_image(self, image_file_name):
         """Create builder-hex0 disk image"""
-        run(os.path.join('sysa', 'stage0-posix', 'src',
-                         'bootstrap-seeds', 'POSIX', 'x86', 'hex0-seed'),
-            os.path.join('kernel-bootstrap', 'builder-hex0-x86.hex0'),
-            image_file_name)
+        shutil.copyfile(os.path.join('kernel-bootstrap', 'builder-hex0-x86-stage1.bin'), image_file_name)
 
         with open(image_file_name, 'ab') as image_file:
+            current_size = os.stat(image_file_name).st_size
+            while current_size != 510:
+                image_file.write(b'\0')
+                current_size += 1
+            image_file.write(b'\x55')
+            image_file.write(b'\xAA')
+
+            # Append stage2 hex0 source
+            with open(os.path.join('kernel-bootstrap', 'builder-hex0-x86-stage2.hex0')) as infile:
+                image_file.write(infile.read().encode())
+            # Pad to next sector
+            current_size = os.stat(image_file_name).st_size
+            while current_size % 512 != 0:
+                image_file.write(b'\0')
+                current_size += 1
             self.append_srcfs(image_file)
 
         current_size = os.stat(image_file_name).st_size
@@ -213,6 +225,6 @@ class SysA(SysGeneral):
 
         # fill file with zeros up to desired size, one megabyte at a time
         with open(image_file_name, 'ab') as image_file:
-            while current_size < 1008 * megabyte:
+            while current_size < 16384 * megabyte:
                 image_file.write(b'\0' * megabyte)
                 current_size += megabyte