config_distro_bootcmd: Avoid scanning usb twice (under some circumstances)

When using usb-keyboard support, the preboot env variable must be set to a
command to scan usb, so that the keyboard is available to interrupt autoboot.

The logical command to add when using config_distro_bootcmd.h is
"run usb_init", as that does a "setenv usb_need_init false" which should avoid
a second scan when booting from usb.

However this does not work because config_distro_bootcmd sets
bootcmd to "setenv usb_need_init; ...".

This is not necessary "if ${usb_need_init}" will evaluate to true just as well
if usb_need_init is not set at all. So drop the BOOTENV_SET_USB_NEED_INIT
macro and calling of it, thereby fixing the double usb-scan.

While at it do the same for scsi_need_init which was modelled after the usb
code.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>