**/linux-*: update usage of downstreamkernel_prepare helper

device/testing/linux-alcatel-pop7lte/APKBUILD

device/testing/linux-amazon-tank/APKBUILD

device/testing/linux-amazon-thor/APKBUILD

device/testing/linux-asus-duma/APKBUILD

device/testing/linux-asus-t00f/APKBUILD

device/testing/linux-asus-tf101/APKBUILD

device/testing/linux-asus-z00t/APKBUILD

device/testing/linux-bq-chaozu/APKBUILD

device/testing/linux-chuwi-hi10plus/APKBUILD

device/testing/linux-fairphone-fp1/APKBUILD

device/testing/linux-fairphone-fp2/APKBUILD

device/testing/linux-finepower-f1/APKBUILD

device/testing/linux-fujitsu-m532/APKBUILD

device/testing/linux-google-crosshatch/APKBUILD

device/testing/linux-google-glass/APKBUILD

device/testing/linux-gp-peak/APKBUILD

device/testing/linux-hisense-m470bsa/APKBUILD

device/testing/linux-htc-a5ul/APKBUILD

device/testing/linux-htc-bravo/APKBUILD

device/testing/linux-htc-evita/APKBUILD