toolchain.inc: Update 'required toolchain' error text

The old text said:
*** building <STAGE> without the required toolchain.  Stop.

Where <STAGE> could be any of the coreboot stages - bootblock, verstage,
ramstage, romstage.

This error message was very misleading though, because what it actually
meant was that it didn't know what architecture was required to build
the stage, not that the toolchain was missing.
Update the text to better reflect the actual issue, and to give the
user a hint as to what to look for:
*** The toolchain architecture for <STAGE> is unknown.
*** Check your .config file for CONFIG_ARCH_<STAGE>_* settings.  Stop.

Change-Id: Ic2a4f60c1f25e0f5e1ebde76781bcb8da0987d82
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16024
Tested-by: build bot (Jenkins)
Reviewed-by: Omar Pakker

toolchain.inc

@@ -111,7 +111,8 @@ set_stage_toolchain= \
 # @2: compiler set to be used
 # e.g.: smm special class uses i386 as compiler set
 define create_class_compiler
-$(if $(2),,$(error building $(1) without the required toolchain))
+$(if $(2),,$(warning *** The toolchain architecture for $(1) is unknown.) \
+	$(error Check your .config file for CONFIG_ARCH_$(1)_* settings))
 CC_$(1) := $(CC_$(2))
 LD_$(1) := $(LD_$(2))
 NM_$(1) := $(NM_$(2))