hw/arm/boot: Make user not specifying a kernel not an error

Typically ARM boards will have some kind of flash which might contain
a boot ROM; it's therefore a valid use case to provide only an
image for the boot ROM and not require QEMU's internal boot loader
at all. Remove the fatal error if -kernel isn't specified.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1379980897-21277-2-git-send-email-peter.maydell@linaro.org
