[yocto] docs: standardizing on "+=" versus "_append" for clarity

[Robert P. J. Day]

  when i teach OE/YP classes, a moderately regular question is when to
use "+=" versus "_append", given that some of the docs confusingly
bounce back and forth for no apparent reason.

  eg: in-progress kernel dev manual, section 2.2.3, contains snippets:

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
     SRC_URI += "file://defconfig"

     ... snip ...

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
     SRC_URI += "file://8250.cfg"

but down in section 2.7, we see:

     SRC_URI_append = " file://test.scc"

     ... snip ...

     KERNEL_FEATURES_append = " test.scc"

and in section 3.2:

     KERNEL_FEATURES += "features/netfilter/netfilter.scc"

     ... snip ...

     KERNEL_FEATURES_append_qemux86 = " cfg/sound.scc"

i understand that there are times when you *want* _append to delay
processing until the end, and i understand that conditional appending
with OVERRIDES requires the "_append" syntax, but other than these
special cases, is there an encouraged standard to use "+=" if either
syntax will do?

rday

p.s. i can assure people that it's exactly this kind of bouncing back
and forth that confuses readers, who suddenly think, "wait a minute, i
thought i understood this, but maybe i don't."

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
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