zephyr: direct inclusion of the hooks file

This patch removes CONFIG_BOOT_IMAGE_ACCESS_HOOKS_FILE option
which was used to point to hooks implementation file.
It is better that the project customization is up to add required
files instead. This gives more flexibility in provisioning these
sources (there may be multiple files).
This also simplifies Kconfig options.

In order to keep possibility for build-test the Hooks feature
hooks_sample.c will be included into the build if commandline
will specify -DTEST_BOOT_IMAGE_ACCESS_HOOKS=Y option.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>