Improve Makefile to unify building and installation

Currently, there are duplicated recipes and documentation with regards to how to install the mender client on a device.

List follows:

• meta-mender
• Buildroot
• mender-convert
• Blog post
• Mender client debian package
• OpenWRT conversion scripts (not available publicly yet)

To avoid duplicity of information and ease maintenance, we want to transfer the "install" knowledge to the `mender` client repo, specifically to its Makefile `install` target.

The idea would be to reduce the user instructions to something simple like:

go get github.com/mendersoftware/mender
cd $GOPATH/src/github.com/mendersoftware/mender
make
sudo make install

See the full discussion here

Acceptance criteria:

• Include both demo and production configuration templates in make install (e.g. mender.conf.demo, mender.conf.production)
• mender.conf.production uses Hosted Mender by default
• there is a documented (in README.md) way to cross-compile the Mender client during make
• README.md updated for make install