qemu.bbclass: clarify QEMU_EXTRAOPTIONS

QEMU_EXTRAOPTIONS is a way to add PACKAGE_ARCH specific options to the
qemu_run_binary qemu commandline. The base QEMU_EXTRAOPTIONS variable
(ie without a PACKAGE_ARCH suffix) is not used, so defining it, either
directly or via an over-ride has no effect.

Although previously an over-ride for _armv7a was used, it did nothing
for most armv7a builds, which typically use PACKAGE_ARCH values such
as "cortexa9hf-neon". In practice this worked OK since without a -cpu
option, qemu-arm will default to emulating a CPU which supports all
required architecture levels.

qemu-arm (v2.5.0) with no -cpu option has been confirmed to
successfully run binaries built for armv7ve.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>

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