Support external build directory
When running inside a QEMU microvm with the source directory shared via 9pfs, the kernel build process fails > Error: Could not mmap file: vmlinux Thus, we need to run the build in a path on a local filesystem. To support this, the Makefile now supports an `O` option, and all the build scripts have been adjusted to make use of it as needed. Since building in a local filesystem would ultimately discard the final artifacts when the VM terminates, we need yet a different location for the files we want to keep. The `IMAGESDIR` option can be used to specify this path. This path can be on a shared filesystem, thus saving the artifacts outside the microvm.
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