tools/bootconfig: specify LDFLAGS as an argument to CC

Explicitly specify LDFLAGS as an argument to CC so that this can be
overridden by the user.

Link: https://lore.kernel.org/all/20250328183858.1417835-3-bmasney@redhat.com/



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