Enable 'early_microcode' by default

Enable 'early_microcode' by default

We initially enabled it for Haswell TSX bug (mga#16657)

Now there is also Meltdown and Spectre security issues,
and more microcode issues will most likely show up...

So the sane default for 'early_microcode' to have it enabled,
as theese changes must be done early in boot process to take
effect as intended.

Update documentation accordingly.

Reference: https://bugs.mageia.org/show_bug.cgi?id=16657

Signed-off-by: Thomas Backlund <tmb@mageia.org>
Signed-off-by: Neal Gompa <ngompa13@gmail.com>