Makefile: fix config-devices.mak generation

The logic of detecting changes in default-configs/*.mak is
flawed as can be demonstrated by 'touch default-configs/*.mak'
followed by make. This results in a message claiming that user
made changes to the */config-devices.mak files.

Fix by separating the detection of changes made by the user and
changes in the default-configs.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>