build: make <subdir>/install opt-in, use it for target/ only

Fixes buildbot errors on running make target/install or
toolchain/install

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/Makefile

@@ -6,6 +6,7 @@
 #
 curdir:=target
 
+$(curdir)/subtargets:=install
 $(curdir)/builddirs:=linux sdk imagebuilder toolchain
 $(curdir)/builddirs-default:=linux
 $(curdir)/builddirs-install:=linux $(if $(CONFIG_SDK),sdk) $(if $(CONFIG_IB),imagebuilder) $(if $(CONFIG_MAKE_TOOLCHAIN),toolchain)