Allow this test script to be run from within src/tools/boot dir, and create
the temporary image in $TMPDIR.

Allow the script to be run from the src/tools/boot directory by using make
-V SRCTOP to find the top of the tree, because this script is handy for
quick smoke-testing of loader changes, as well as being useful in CI testing.

Also, use a temp directory in $TMPDIR to assemble the boot image, and write
the boot log file to $TMPDIR. Arrange to have the temporary image clean
itself up, but leave the log file in $TMPDIR for post-mortem analysis of
failures when the script is run interactively.

Differential Revision: https://reviews.freebsd.org/D19876