[OE-core] RFC: Secondary Toolchain

[Mark Hatle]

We have an issue where we'd like to have an alternative toolchain (assembler, 
linker, compiler) available for our customers to selectively use.  However, 
before we just go off and implement something, I'd like at least some basic 
consensus on what the best practice is for doing this work.  Below is my attempt 
at an early proposal.

Background
----------------

Many companies have commercial / highly optimized toolchains for specific 
purpose, such as ICC from Intel, LLVM, ARM specific toolchain, etc..  For 
(potentially) better performance with some applications a mechanism to both 
provide the hooks for the alternative toolchain as well as a way to active it 
per-package is desired.

This work assumes that the third party toolchain is generally compatible with 
the idea of sysroots, linking to the libc provided by OE, and generally 
compatible with GNU conventions.

However, as part of the third party toolchain, it may not be GNU compatible. 
This means many Open Source applications simply may not work with this 
toolchain.  That means that we need to have a way for a toolchain to blacklist 
(or whitelist) specific recipes.  This way only supported components can be 
built by the user, avoiding numerous complaints.

Currently OE has a method to generate an SDK based on the GNU toolchain.  We 
would like to be able to also export the external toolchain along with the SDK, 
effectively providing both the GNU toolchain and the third party toolchain using 
the common sysroot.

We need a way to active the third party toolchain on a per-package basis.  This 
activation will need to use the existing sysroot, but be able to pass different 
C, C++, LD, AS and other flags as specified by the third party toolchain.

Finally third party toolchains should be implemented as layers that can easily 
plug into OE.

Implementation
---------------------

Add an OVERRIDE to specify the alternative toolchain.  Can this be done within 
the layer by doing something like:

OVERRIDE_append = ":toolchain-${TOOLCHAIN}"

TOOLCHAIN = "gnu" (or "icc")

To activate the toolchain you would use things like:

TOOLCHAIN_pn-bash = 'icc'

To define the correct behavior for the toolchain, the following would need to be 
defined (with _toolchain-<toolchain>):

TARGET_CPPFLAGS
TARGET_CFLAGS
TARGET_CXXFLAGS
TARGET_LDFLAGS
CC
CXX
F77
CPP
LD
CCLD
AR
AS
RANLIB
STRIP
OBJCOPY
OBJDUMP
NM
FULL_OPTIMIZATIONS
DEBUG_OPTIMIZATION

Is anyone aware of any other items that may require additional items?  Will the 
above actually work?  Using the override of the TOOLCHAIN_… will that actually 
change the override values or do we get stuck?

Comments/suggestions appreciated!
--Mark