Alpine does not build the linux-virt flavor for riscv64, so --kernel
without a path failed there with 'linux-virt package not found'. Scan
the APKINDEX once for both flavors and prefer linux-virt, taking
linux-lts when the virt flavor is missing.
Also store the cached kernel decompressed: riscv64 and aarch64 ship
their image gzipped (Image.gz), and QEMU's riscv -kernel loader
understands only ELF, uImage and raw images, so the gzipped image hung
at boot after the OpenSBI banner.
vhaudiquet
5dd9dff76a