x86: qemu: Fix non-working ramboot and nfsboot environment variables

With qemu-x86 starting to use config_distro_bootcmd, the pre-defined
ramboot and nfsboot commands do not work any more. This is caused by
undefined environment variable 'ramdiskaddr' that was previously set
in CONFIG_EXTRA_ENV_SETTINGS but later CONFIG_EXTRA_ENV_SETTINGS was
redefined for distro boot.

Update the x86 generic CONFIG_EXTRA_ENV_SETTINGS to consider distro
boot, and remove the one in qemu-x86.h.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>