Changes

contrib/jenkins: remove broken chroot + qemu code

The new rpi4 jenkins node uses a 64 bit arm OS, hence it returns
aarch64 for $(uname -m) / $(arch). This is not expected in the
INSIDE_CHROOT block and so it attempts to create the chroot and fails.

Upon closer look, the mirror where it would download images from is not
online anymore, and it looks like this whole block has not been
executed in years (we always test the arm builds on native arm machines
nowadays, and run all builds in docker).

Replace the whole block with a simple check that ensures when arm flags
are used, that the build is actually running on an arm machine.

Related: OS#6549
Change-Id: I2e204e4a0a7dfcf32f011ed6fb403b564f8155a7