main/linux-rpi: fix config prompt during cross-compilation

Open Oliver Kuckertz requested to merge mologie/aports:fix-linux-rpi-crossbuild into master

The ARCH parameter was missing from the syscconfig call, which resulted in incomplete configuration during cross-compilation and broke the build with an interactive "Restart config..." prompt when building _dev packages.

Also adds a check to ensure that _carch is set, so that an unexpected CARCH value will be caught early and not trigger similar unwanted behavior.

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