linux-base: Support LINUX_CONFIG to add any local config files
linux-base: Support LINUX_CONFIG to add any local config files

Previously, linux-base only supports to get kernel configs from
LINUX_DEFCONFIG and .config files given in SRC_URI, but
users might want to give config files from other places like sysroot.
LINUX_CONFIG can be used to handle such requirement.

Signed-off-by: Kazuhiro Hayashi <5579fccaa800baf92910671700646e6b27853b34@gmail.com>

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