Use tarball for kernel source

[amclain]

This PR switches the kernel source from a git repo to a tarball to speed up the download time.

Booting Nerves from an x86 system was not tested.

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Note that after running make savedefconfig, the following was automatically updated in nerves_defconfig that is not included in this PR:

-BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
-BR2_TOOLCHAIN_EXTERNAL_URL="https://github.com/nerves-project/toolchains/releases/download/v1.6.0/nerves_toolchain_x86_64_nerves_linux_musl-linux_${shell uname -m}-1.6.0-63239A0.tar.xz"
-BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX="$(ARCH)-nerves-linux-musl"
-BR2_TOOLCHAIN_EXTERNAL_GCC_11=y
-BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_19=y
-BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL=y
-BR2_TOOLCHAIN_EXTERNAL_CXX=y
-BR2_TOOLCHAIN_EXTERNAL_OPENMP=y