Image rules: filter packages list via FFilterByBuildFeatures

AddPackageFilesToHaikuImage, AddHaikuImagePackages: For convenience
filter the given package file/package list through
FFilterByBuildFeatures.
EnableBuildFeatures: deal with duplicate invocations
BuildFeatures: print the packaging arch in error messages
HaikuPorts/x86_gcc2: add haikuwebkit and xz_utils
DefineDefaultBuildProfiles: simplify things a bit
PreprocessPackageInfo: add substitution

... HAIKU_SECONDARY_PACKAGING_ARCH_SUFFIX. It's "_<arch>" for a
secondary architecture and "" for the primary one.
BuildFeatures: re-add webkit build feature
Add webpositive package to image