mbedtls_dhm_get_value can be seen as either a copy function or a getter
function. Given the name and the semantics, it's more of a getter, even if
it "gets" by doing a copy. Therefore, put the context first, and the
selector next, leaving the output for last.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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