busybox: always start do_compile with orig config files

When compiling busybox a second time (e.g. with `compile -f`), busybox
can use an altered autoconf.h file for compiling, which can ultimately
produces different and unwanted binaries.

This can produce errors like this one:

ERROR: busybox-1.35.0-r0 do_package: Error executing a python function in exec_func_python() autogenerated:
The stack trace of python calls that resulted in this exception/failure was:
File: 'exec_func_python() autogenerated', lineno: 2, function: <module>
     0001:
 *** 0002:ptest_update_alternatives(d)
     0003:
File: '…/poky/meta/classes/ptest.bbclass', lineno: 100, function: ptest_update_alternatives
     0096:        for alt_name, alt_link, alt_target, _ in alternatives:
     0097:            # Some alternatives are for man pages,
     0098:            # check if the alternative is in PATH
     0099:            if os.path.dirname(alt_link) in bin_paths:
 *** 0100:                os.symlink(alt_target, os.path.join(ptest_bindir, alt_name))
     0101:}
     0102:
     0103:do_configure_ptest_base[dirs] = "${B}"
     0104:do_compile_ptest_base[dirs] = "${B}"
Exception: FileExistsError: [Errno 17] File exists: '/bin/busybox.suid' -> '…/busybox/1.35.0-r0/package/usr/lib/busybox/ptest/bin/login'

This happens because ALTERNATIVE:busybox contains `/bin/login` twice,
initially that's because `/bin/login` is present in both
busybox.links.suid and busybox.links.nosuid. The reason for that is
because of the altered autoconf.h.

Steps to reproduce above error:

<add ptest to distro configs>
bitbake busybox -c clean
bitbake busybox -c package -f
bitbake busybox -c compile -f
bitbake busybox -c package -f

This patch guards against potential bugs by:

- making a backup of .config and autoconf.h that have matching
  timestamps.
- make sure do_compile always starts with these files.
- restore .config and autoconf.h at the end of do_compile.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>