abuild.conf: simplify. only set CHOST and let abuild detect the rest

abuild.conf

-CARCH=x86
-CBUILD=i486-alpine-linux-uclibc
+CHOST=i486-alpine-linux-uclibc
 
 # for x86_64 we want:
-# CARCH=x86_64
-# CBUILD=x86_64-alpine-linux-uclibc
+# CHOST=x86_64-alpine-linux-uclibc
 
 # for powerpc we want:
-# CARCH=powerpc
-# CBUILD=powerpc-alpine-linux-uclibc
-
-CHOST=$CBUILD
-CTARGET=$CHOST
+# CHOST=powerpc-alpine-linux-uclibc
 
 export CFLAGS="-Os -fomit-frame-pointer"
 export CXXFLAGS="$CFLAGS"