core-image-sato: Fix runqemu error for qemuarmv5

When attempting to execute runqemu on qemuarmv5, the following error is
encountered:

runqemu - ERROR - Failed to run qemu: qemu-system-arm: versatilepb: memory size must not exceed 256MB

To work around this, limit the QB_MEM size for qemuarmv5, similar to
what is being done for qemumips.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>