Move Config.in.mirrors out of target/device

Having Config.in.mirrors (which also to select various download sites)
inside target/device sounds strange. This commit moves the contents of
Config.in.mirrors directly into the main Config.in file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>