extend config validation to all build components

while `lb build` ran the config validation check, spotting invalid configs
and stopping with an error, the major build stages if executed directly did
not, nor did the component stages.

here we move execution of the validation function into the common init
function, with an exported variable used to indicate that validation has
been performed. thus validation is performed no matter what part of the
build system you execute, but only once.

Gbp-Dch: Short

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