nios2:allmodconfig builds fail with

make[1]: *** No rule to make target 'arch/nios2/boot/dts/""',
	needed by 'arch/nios2/boot/dts/built-in.a'.  Stop.
make: [Makefile:1868: arch/nios2/boot/dts] Error 2 (ignored)

This is seen with compile tests since those enable NIOS2_DTB_SOURCE_BOOL,
which in turn enables NIOS2_DTB_SOURCE. This causes the build error
because the default value for NIOS2_DTB_SOURCE is an empty string.
Disable NIOS2_DTB_SOURCE_BOOL for compile tests to avoid the error.

Fixes: 2fc8483f ("nios2: Build infrastructure")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>