Parallel make issue

[osresearch]

I think our parallel make issue might be a Make bug, rather than a makefile bug. Here's a minimal test case inspired by this answer https://stackoverflow.com/a/47951465 :

all: e
.INTERMEDIATE: d

e: a b c
    cat $^ > $@
a b c: d
d: i1 i2
    cat i1 i2 > a
    cat i1 i2 > b
    cat i1 i2 > c

i1 i2:
    touch $@

In a single threaded make this does the right thing. it creates i1 and i2, then cats them to make a b and c, then cats a b c to make e.

/tmp: make
touch i1
touch i2
cat i1 i2 > a
cat i1 i2 > b
cat i1 i2 > c
cat a b c > e

If e doesn't exist, it is recreated correctly in a parallel make:

/tmp: rm i1 i2 e a b c
/tmp: make -j4
touch i1
touch i2
cat i1 i2 > a
cat i1 i2 > b
cat i1 i2 > c
cat a b c > e

But if e already exists, make will regenerate a, b, and c, but not update e on the first run. It does recreate e on a second run:

/tmp: rm i1 i2
/tmp: make -j4
touch i1
touch i2
cat i1 i2 > a
cat i1 i2 > b
cat i1 i2 > c
/tmp: make -j4
cat a b c > e

[osresearch]

Reworking the parallel dependencies and allowing separate foo_dir and foo_base_dir variables for things that do per-board builds (like Linux and coreboot).
a772b27

[osresearch]

PR #395 should fix this.