configs/stm32mp157c_dk2: add hashes

configs/stm32mp157c_dk2: add hashes

Now that the configurations of boards stm32mp157a-dk1 and stm32mp157c-dk2
have the same software versions, it is possible to share the hash files as
well.
The patch uses the hash files for Arm Trusted Firmware, Linux, Linux
headers and U-Boot of stm32mp175a_dk1 also for stm32mp175c_dk2, and then
enables BR2_DOWNLOAD_FORCE_CHECK_HASHES. With this, we can now drop the
defconfig from .checkpackageignore.

Signed-off-by: Dario Binacchi <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>