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.kaem

@@ -0,0 +1,16 @@
+# SPDX-FileCopyrightText: 2023 Richard Masters <grick23@gmail.com>
+# SPDX-License-Identifier: MIT
+cd src
+tcc -static -m32 -march=i386 -std=c89 -I../../tcc/tcc-0.9.27/include -o ${bindir}/kexec-linux kexec-linux.c
+cd ..
+
+# Checksums
+if match x${UPDATE_CHECKSUMS} xTrue; then
+    sha256sum /usr/bin/kexec-linux
+    sha256sum -o ${pkg}.checksums \
+        /usr/bin/kexec-linux
+
+    cp ${pkg}.checksums ${srcdir}
+else
+    sha256sum -c ${pkg}.checksums
+fi