toolchain: Provide abstraction for recipe specific toolchain selection

This implements a toolchain selection mechanism, defaulting to gcc
as per the existing defaults.

Introduce a variable called TOOLCHAIN, which denotes the familiar
name for toolchain e.g. "gcc" which selects GNU compiler + binutils
as default C/C++ toolchain or "clang" which will use LLVM/Clang Compiler

TOOLCHAIN variable has a global fallback to "gcc" in configuration
metadata. A distro can switch to using say "clang" as default system
compiler by defining

TOOLCHAIN ?= "clang"

In local.conf or other distro specific global configuration metadata

It is also selectable at recipe scope, since not all packages are
buildable with either clang or gcc, a recipe can explicitly demand
a given toolchain e.g. glibc can not be built with clang therefore
glibc recipe sets.

TOOLCHAIN = "gcc"

Based on ideas/work by Khem Raj <raj.khem@gmail.com>

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: rpurdie

meta/classes-global/base.bbclass

@@ -19,6 +19,12 @@ PACKAGECONFIG_CONFARGS ??= ""
 
 inherit metadata_scm
 
+TOOLCHAIN ??= "gcc"
+TOOLCHAIN:class-native ??= "gcc"
+TOOLCHAIN:class-nativesdk ??= "gcc"
+inherit toolchain/gcc-native
+inherit_defer toolchain/${TOOLCHAIN}
+
 def lsb_distro_identifier(d):
     adjust = d.getVar('LSB_DISTRO_ADJUST')
     adjust_func = None

meta/conf/toolchain/clang.inc renamed to meta/classes/toolchain/clang.bbclass

@@ -834,9 +834,6 @@ include conf/licenses.conf
 require conf/sanity.conf
 include conf/bblock.conf
 
-require toolchain/gcc.inc
-require toolchain/build-gcc.inc
-
 ##################################################################
 # Weak variables (usually to retain backwards compatibility)
 ##################################################################

meta/conf/toolchain/build-gcc.inc renamed to meta/classes/toolchain/gcc-native.bbclass

@@ -9,6 +9,8 @@ TARGET_ARCH[vardepvalue] = "${TARGET_ARCH}"
 INHIBIT_DEFAULT_DEPS = "1"
 INHIBIT_AUTOTOOLS_DEPS = "1"
 
+TOOLCHAIN = "gcc"
+
 SRC_URI += "file://0002-binutils-cross-Do-not-generate-linker-script-directo.patch"
 
 # Specify lib-path else we use a load of search dirs which we don't use

meta/conf/toolchain/gcc.inc renamed to meta/classes/toolchain/gcc.bbclass

@@ -11,6 +11,7 @@ PN = "clang-cross-${TARGET_ARCH}"
 require common-clang.inc
 require common-source.inc
 inherit cross
+TOOLCHAIN = "clang"
 DEPENDS += "clang-native virtual/cross-binutils"
 
 #INHIBIT_PACKAGE_STRIP = "1"

meta/conf/bitbake.conf

@@ -11,6 +11,7 @@ PN = "clang-crosssdk-${SDK_SYS}"
 require common-clang.inc
 require common-source.inc
 inherit crosssdk
+TOOLCHAIN = "clang"
 DEPENDS += "clang-native nativesdk-clang-glue virtual/nativesdk-cross-binutils virtual/nativesdk-libc"
 
 do_install() {