CFG_LIBDIR specified by target triples in platform.mk

mk
- CFG_LIBDIR changed to specify with target triples like CFG_LIB_DIR_$(target), following other naming convention like CFG_SRC_DIR, CFG_LIB_NAME_$(target)

configure : 
- rename ambiguous CFG_LIBDIR to CFG_DEFAULT_LIB_DIR, which is used filesearch.rs. 
- CFG_DEFAULT_LIB_DIR is associated with CFG_LIB_DIR_$(target) in platform.mk