package/mysql: unconditionally define host variables

To build the host-mysql, we only build parts of the source, just the
strictly minimum required to then cross-compile it.

However, the host variables (conf opts, build and install cmds) are only
defined when the mysql server is enabled in the configuration.

So, this breaks:
    make defconfig; make host-mysql

Even though it is not much use to have that partial host-mysql on its
own, it is still very interesting to be able to build it, if at least
for testing changes in the core package infrastructures (like new step
hooks or the likes...)

Move the definitions of the host variant out of the server conditional
block.

[Peter: add comment about what we build and why as suggested by Arnout]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Marcelo Gutiérrez(UTN/FRH) <kuyurix@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>