This patch from Ian Cambell adds the generic support code:
* CONF.sh: Add $(VARIANTS) configuration variable.
* eash-build.sh: Add command line parameter to enable variants.
* Makefile: Define VARIANT_xxx when preprocessing package list.
* boot/?/common.sh: Add a function for checking if a variant is enabled.
* generate_di_list: Allow variant overrides in udeb exclusion list.
Variant support is documented in docs/README.variants
The intention is to use this support to add support for installing a
Xen guest from an ISO image.
The approach to use varriants was originally suggested by Frans Pop.
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