linux-postmarketos APKBUILD should patch kernel source with latest maint. release patchset

Alpine Linux's kernel APK will download the 4.x source, and then download/apply the latest 4.x.y patchset. This saves build systems from having to download the full 4.x.y source (~90-100MB) each time a new maintenance release comes out. Instead, they can download the 4.x source once, then download each maintenance patchset as it is released.

This mechanism was partially implemented in the linux-postmarketos APKBUILD, but did not actually download the maint. release patchset so it always installed the 4.x kernel regardless of the pkgver (e.g. only installed 4.12 despite pkgver=4.12.4)