[posix-app] add HOST flag for cross compiling (#3991)

This commit add 'HOST' to provide the option for changing compile
target host for OpenWRT.

src/posix/Makefile-posix

@@ -78,7 +78,7 @@ configure_OPTIONS                   = \
 
 # Platform specific switches
 
-ifeq (${DAEMON},1)
+ifeq ($(DAEMON),1)
 configure_OPTIONS              += --enable-posix-app-daemon
 endif
 
@@ -88,6 +88,10 @@ COMMONCFLAGS                   += \
     $(NULL)
 endif
 
+ifneq ($(HOST),)
+configure_OPTIONS              += --host=$(HOST)
+endif
+
 ifeq ($(PLATFORM_NETIF),1)
 configure_OPTIONS              += --enable-platform-netif
 endif