docs/manual: add section about build time graphing

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GRAPH_OUT=svg make graph-depends

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Graphing the build duration

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[[graph-duration]]

When the build of a system takes a long time, it is sometimes useful

to be able to understand which packages are the longest to build, to

see if anything can be done to speed up the build. In order to help

such build time analysis, Buildroot collects the build time of each

step of each package, and allows to generate graphs from these data.

To generate the build time graph after a build, run:

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make graph-build

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This will generate a set of files in +output/graphs+ :

* +build.hist-build.pdf+, an histogram of the build time for each

  package, ordered in the build order.

* +build.hist-duration.pdf+, an histogram of the build time for each

  package, ordered by duration (longest first)

* +build.hist-name.pdf+, an histogram of the build time for each

  package, order by package name.

* +build.pie-packages.pdf+, a pie chart of the build time per package

* +build.pie-steps.pdf+, a pie chart of the global time spent in each

  step of the packages build process.

This +graph-build+ target requires the Python Matplotlib and Numpy

libraries to be installed (+python-matplotlib+ and +python-numpy+ on

most distributions), and also the +argparse+ module if you're using a

Python version older than 2.7 (+python-argparse+ on most

distributions).

By default, the output format for the graph is PDF, but a different

format can be selected using the +GRAPH_OUT+ environment variable. The

only other format supported is PNG:

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GRAPH_OUT=png make graph-build

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