boot: zephyr: add integration platforms to the allow list

The platforms that are listed as integration platforms in
the Zephyr boot sample need to be included in the platform
allow list, otherwise zephyr CI will throw an error. This
commit removes the common list of integration platforms and
adds integration platform entries in each test variant, in
accordance with the allow-list in each of the variants.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>