split platform definitions out of mk/platform.mk

The goal here is to make it easier to add new platform definitions,
especially when the additions are programmatic (ie: in build scripts).

configure

@@ -781,7 +781,7 @@ CFG_PREFIX=${CFG_PREFIX%/}
 CFG_MANDIR=${CFG_MANDIR%/}
 CFG_HOST="$(echo $CFG_HOST | tr ',' ' ')"
 CFG_TARGET="$(echo $CFG_TARGET | tr ',' ' ')"
-CFG_SUPPORTED_TARGET="$(grep ^CC_*=* ${CFG_SRC_DIR}mk/platform.mk | sed -e 's/^CC_//' -e 's/\([^=]*\).*/\1/' | xargs)"
+CFG_SUPPORTED_TARGET="$(ls ${CFG_SRC_DIR}mk/cfg)"
 
 # copy host-triples to target-triples so that hosts are a subset of targets
 V_TEMP=""