restore include of make.conf to orginal location

[sjg]

Summary

Including make.conf early breaks users who have
non-trivial things there.

Move its include back to where it was, and include
local.sys.env.mk early - which we specifically define to
only do things safe at that early point.

Might as well add a corresponding src-env.conf to
include via src.sys.env.mk

[sjg]

simplify include of SRC_ENV_CONF

[avg]

Looks like this differential was generated from a tree that differed from a vanilla head.
When I ran arc patch D2860 it couldn't find share/mk/src.sys.env.mk.

[sjg]

Avg: I did

svn cp local.sys.mk local.sys.env.mk
svn cp src.sys.mk src.sys.env.mk

so no you won't find them in svn yet

[imp]

First blush, this looks good to me. I'll sleep on it to see if anything bubbles up while the rest of my brain is off...

[avg]

In D2860#55283, @sjg wrote:

svn cp local.sys.mk local.sys.env.mk
svn cp src.sys.mk src.sys.env.mk

arc patch D2860 still failed to apply cleanly, so I can't test.