编译OpenWrt package的PKG_TARGETS:=bin和PKG_TARGETS:=ipkg区别
PKG_TARGETS:=bin
define Package/$(PK_NAME)/install
$(1) 为$(BINDIR)/$(PK_NAME)，如bin/goldfish/goldfish-qemu
endef
并且不能反复编译make xx/install，只会在总体make时才会编译
而PKG_TARGETS:=ipkg
define Package/$(PK_NAME)/install
$(1) 为staging/root_xxx
endef
