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/kexec-linux/kexec-linux.checksums

@@ -0,0 +1 @@
+c0993adf00bd2c110539a310246f0e50d727e09701ed187827c71c64021431f4  /usr/bin/kexec-linux