tools/bootconfig: allow overriding CFLAGS assignment

Allow overriding the CFLAGS assignment so that the user can pass in
an outside value.

Link: https://lore.kernel.org/all/20250328183858.1417835-2-bmasney@redhat.com/



Signed-off-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>