pmb.aportgen.linux: fix generated apkbuild when using qcdt

pmbootstrap generates wrong install command in package() block in kernel APKBUILD when device uses qcdt:

install -Dm644 "arch/$_carch/boot"/dt.img "$pkgdir"/boot/dt.img

which is wrong, because we output dt.img to "$_outdir/arch/$_carch/boot"/dt.img, then we try to copy it from "arch/$_carch/boot"/dt.img and fail because file does not eхist there.

Fixes !1893 (merged)