[Buildroot] Handling of rpi-firmware blobs broken for certain cases

[Simon van der Veldt]

Good evening everyone,

I just ran into an issue where the .elf and .dat files copied into
/boot by buildroot when building for a Raspberry Pi are incorrect.

Overview of the problem using current master (or 2018.11.2 which is
the same apart from the version):
- Put gpu_mem=16 in config.txt (the config file for the videocore of a
raspberry)
- When using <32M GPU memory the videocore needs the so called cut
down versions ("start_cd.elf" and "fixup_cd.dat") of the binary blobs
to boot. By emperical testing it seems at least "start_cd.elf" needs
to be present under that exact name to be able to boot the rpi
- Buildroot has some custom handling for these files
(https://github.com/buildroot/buildroot/blob/master/package/rpi-firmware/rpi-firmware.mk#L40)
and copies the correctly named "source" files from the rpi-firmware
package to the incorrect "start.elf" and "fixup.dat" names
- When trying to boot the RPi won't book (note I've only tested this
with a CM3, not sure if behavior is different for other RPis)

Whilst I'm sure this is solvable I'm wondering why this level of
micro-management of these blobs is necessary? The .elf files are
12.2MB and the .dat files are 28.6KB for all of them.
These files only get copied into the image directory from where one
can then choose which files to include in the actual image using the
genimage config. I don't think copying +-12MB should be a problem.

Also users have the possibility of changing the config.txt file
(assume a writeable boot filesystem of course) which means they might
need different versions of these start and fixup files and by
extension multiple/all different versions of these blobs need to be
present.

With all of this said my suggestion would be to simplify how this is
handled in buildroot by just copying all *.elf and *.dat files in the
boot directory of rpi-firmware to the image directory.
Then the person building the image can choose which of these (specific
ones or all of them) to include using the genimage config.

An example/suggestion for a fix can be found here
https://github.com/simonvanderveldt/buildroot/commit/16880713c2c03ad50e8cfe7bf05331718d52742e
I've tested this and it works like a charm :)

Kind regards,
Simon