mk: Fix bootstrapping cross-hosts on beta

The beta builds are currently failing, unfortunately, due to what is presumably
some odd behavior with our makefiles. The wrong bootstrap key is being used to
generate the stage1 cross-compiled libraries, which fails the build.
Interestingly enough if the targets are directly specified as part of the build
then it works just fine! Just a bare `make` fails...

Instead of trying to understand what's happening in the makefiles instead just
tweak how we configure the bootstrap key in a way that's more likely to work.

Author: alexcrichton

mk/target.mk

@@ -42,6 +42,23 @@ $(foreach host,$(CFG_HOST), \
    $(foreach crate,$(CRATES), \
     $(eval $(call RUST_CRATE_FULLDEPS,$(stage),$(target),$(host),$(crate)))))))
 
+# $(1) stage
+# $(2) target
+# $(3) host
+define DEFINE_BOOTSTRAP_KEY
+BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3) := $$(CFG_BOOTSTRAP_KEY)
+ifeq ($(1),0)
+ifeq ($(3),$$(CFG_BUILD))
+BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3) := $$(CFG_BOOTSTRAP_KEY_STAGE0)
+endif
+endif
+endef
+
+$(foreach host,$(CFG_TARGET), \
+ $(foreach target,$(CFG_TARGET), \
+  $(foreach stage,$(STAGES), \
+   $(eval $(call DEFINE_BOOTSTRAP_KEY,$(stage),$(target),$(host))))))
+
 # RUST_TARGET_STAGE_N template: This defines how target artifacts are built
 # for all stage/target architecture combinations. This is one giant rule which
 # works as follows:
@@ -65,12 +82,9 @@ $(foreach host,$(CFG_HOST), \
 # $(4) is the crate name
 define RUST_TARGET_STAGE_N
 
-ifeq ($(1),0)
-$$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
-	export RUSTC_BOOTSTRAP_KEY := $$(CFG_BOOTSTRAP_KEY_STAGE0)
-endif
-
 $$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): CFG_COMPILER_HOST_TRIPLE = $(2)
+$$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
+	export RUSTC_BOOTSTRAP_KEY := $$(BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3))
 $$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
 		$$(CRATEFILE_$(4)) \
 		$$(CRATE_FULLDEPS_$(1)_T_$(2)_H_$(3)_$(4)) \
@@ -118,11 +132,8 @@ endef
 # $(4) - name of the tool being built
 define TARGET_TOOL
 
-ifeq ($(1),0)
 $$(TBIN$(1)_T_$(2)_H_$(3))/$(4)$$(X_$(2)): \
-	export RUSTC_BOOTSTRAP_KEY := $$(CFG_BOOTSTRAP_KEY_STAGE0)
-endif
-
+	export RUSTC_BOOTSTRAP_KEY := $$(BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3))
 $$(TBIN$(1)_T_$(2)_H_$(3))/$(4)$$(X_$(2)): \
 		$$(TOOL_SOURCE_$(4)) \
 		$$(TOOL_INPUTS_$(4)) \