Bugfix: some paths created via installing package:lib-armcl-viascons are wrong

Due to the "build" directory living inside "src", environment variables `CK_ENV_LIB_ARMCL_LIB` and `CK_ENV_LIB_ARMCL_INCLUDE` represent broken links.

Please note that these environment are generated by soft:lib.armcl which is shared between packages that use `scons` and packages that don't. We should be careful not to screw up the "dark" half of the use cases.