Skip AS detection when using --enable-external-build

The option exists specifically to allow for configurations
where the build environment is different from the configure
environment.

Change-Id: I95196fa3c49700251d10ff5d256dc7380e39d0c4

build/make/configure.sh

@@ -1207,6 +1207,9 @@ EOF
       check_gcc_machine_option avx
       check_gcc_machine_option avx2
 
+      if enabled external_build; then
+        log_echo "  skipping assembler detection"
+      else
         case "${AS}" in
           auto|"")
             which nasm >/dev/null 2>&1 && AS=nasm
@@ -1225,6 +1228,7 @@ EOF
             ;;
         esac
         log_echo "  using $AS"
+      fi
       [ "${AS##*/}" = nasm ] && add_asflags -Ox
       AS_SFX=.asm
       case  ${tgt_os} in