boot/uboot: needs make >= 4.0

boot/uboot: needs make >= 4.0

Starting with 2020.01, uboot started using the 'undefine' make
directive, which was only introduced with make 4.0.

In the general case, we do not have a way to know if the uboot
selected by the user is older or later than 2020.01, so we have
no way to know before hand if make >= 4.0 is needed or not. As
such, we have no other option than to always require it.

So, use the existing $(BR2_MAKE_HOST_DEPENDENCY) and $(BR2_MAKE),
both of each will ensure that we do use a make that is at least 4.0.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>