external/boot-tests: update boot-test.sh usage for BMC single partition flashing
external/boot-tests: update boot-test.sh usage for BMC single partition flashing

Update boot-test.sh usage to document single partition flashing on BMC
platforms.

Signed-off-by: Andrew Donnellan <efc2d7c347a094a6ec1d918179e819039044c564@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

csmart/skiboot,qemu/skiboot,mikey/skiboot,open-power/skiboot,csmart/skiboot,legoater/skiboot,apopple/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,apopple/skiboot,qemu/skiboot,apopple/skiboot,legoater/skiboot,mikey/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,stewart-ibm/skiboot,open-power/skiboot,mikey/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,csmart/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot