linux: enable MICROCODE_INTEL if intel-microcode is selected

We already turn on kernel features for several packages, so let's do it
for intel-microcode too, otherwise it's impossible to load the microcode
(by means of iucode-tools).

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>