pmbootstrap doesn't automatically pick the right architecture for building when none is specified

Created by: ollieparanoid

Sorry, #935 caused the following regression.

Example: pmbootstrap build linux-samsung-i9100

What should happen:

• pmbootstrap figures out, that this is only available for armhf and start building for armhf

What happens:

• pmbootstrap complains, that it can't be built for x86_64 and that you should specify the architecture. It aborts there.

Workaround: pmbootstrap build --arch=armhf linux-samsung-i9100