Changing SDKMACHINE (which changes SDK_ARCH) shouldn't cause
target task signatures to change. Exclude the dependency
on SDK_ARCH for this reason. It only affects nativesdk builds
and those already account for SDK_ARCH in the build WORKDIR paths.
(From OE-Core rev: dae7c45fac1d877203f173842d43abc4883b808b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>