i2c: Add a CONFIG_I2C master switch to the configuration files

The i2c core and the at24c EEPROM should only be compiled and linked
on the machines that support i2c. Otherwise it's quite strange to see
the at24c-eeprom to be "available" on qemu-system-s390x for example.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1516634853-15883-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>