arch/xtensa: add definitions for the Xtensa overlay

Currently, we have four packages that need to use the Xtensa overlay:
binutils, gcc-initial, gcc-final and gdb. However, they all use three
different ways to check for, and use the overlay. Furthermore, two of
them define the same variable: XTENSA_CORE_NAME is defined both by
binutils and gdb

In order to sanitise that, have the Xtensa architecture export the
appropriate variables: the name of the core and the path to the overlay
tarball. Packages can then check and use those variables.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/arch.mk.xtensa

@@ -0,0 +1,15 @@
+################################################################################
+# Those two variables can be used by packages that need to extract the overlay.
+#
+# ARCH_XTENSA_CORE_NAME   is the name of the Xtensa core; empty if none or the
+#                         standard 'fsf' core.
+# ARCH_XTENSA_OVERLAY_TAR is the path to the overlay tarball; only valid when
+#                         ARCH_XTENSA_CORE_NAME is not empty.
+#
+# Example:
+#   ifneq ($(ARCH_XTENSA_CORE_NAME),)
+#       tar xf $(ARCH_XTENSA_OVERLAY_TAR) -C $(@D) --strip-components=1 gcc
+#   endif
+################################################################################
+ARCH_XTENSA_CORE_NAME = $(call qstrip,$(BR2_XTENSA_CORE_NAME))
+ARCH_XTENSA_OVERLAY_TAR = $(call qstrip,$(BR2_XTENSA_OVERLAY_DIR))/xtensa_$(ARCH_XTENSA_CORE_NAME).tar