Add kernel sources for DSM 6.2.3

[th0ma7]

Motivation: Been waiting for kernel sources for months for DSM-6.2.3 in order to get functional modules to run.
Linked issues: #4144

Checklist

• Build rule all-supported completed successfully
• Package upgrade completed successfully
• New installation of package completed successfully

Changes

• Now only does a modules_prepare instead of building all the modules by default
• Only add + to EXTRAVERSION if kernel is > 4.x

[ymartin59]

Just a minor improvement proposal.

[th0ma7]

@hgy59 and @ymartin59 I decided to go all-in and make all the in-dept changes to the kernel building process as there where way too much churn in the kernel Makefiles ...

Testing still under going to confirm it builds successfully on all 5.x+ DSM.

[th0ma7]

@ymartin59 and @hgy59 May I suggest we remove entirely all kernels < DSM-6.1.
File size are just ridiculous, 1.5-1.6gb where a fraction actually is the kernel sources.
And honestly, I doubt they will ever be used again...

[th0ma7]

Honestly this was brutal... but it ends-up now working quite well.
There is only the obscure case of DSM-4.2 for pcc824 arch which fails to engage on make oldconfig.
Beyond that, I tested every single one of them (and have a feeling of having downloaded the entire sourceforge.net)

Now is pretty much the time to adjust variable names or any other change you see fit.
Otherwise, I think its good for a merge.

[ymartin59]

Great job and big effort, really much more maintainable code !

[ymartin59]

@th0ma7 I am interested in building some USB modules for my model, how should I process?
I searched in wiki but with no luck.
Notice there are old references to "REQ_KERNEL" in https://github.com/SynoCommunity/spksrc/wiki/Compile-and-build-rules
May you please write some detail about kernel module compilation?

[th0ma7]

@ymartin59 I'll look into adjusting the wiki pages to reflect some of the change.
Honestly this PR was overdue for kernel dev to actually work.
There are two approach that can be taken: outside from kernel tree or in-tree.
My PR #4144 is using a mechanism that is out-of-tree which requires the kernel to be prepared first than new code is built against it.

Personally I'm more used to the in-tree by simply adapting the configuration to enable the needed device then building the needed pieces at the directory level or down to the driver specific level such as:

$ make modules_prepare
$ make modules M=drivers/i2c
$ make modules M=drivers/media/i2c
$ make modules M=drivers/media/dvb-frontends -j4
$ make modules M=drivers/media/tuners -j4
$ make modules M=drivers/media/usb/em28xx
$ make modules M=drivers/media -j4

After this PR we should be in good shape to enable this functionality through a wrapper of some sort... more work in perspective.

[th0ma7]

BTW, I guess the configuration adjustment helper used busybox could in a form or another be reused for kernel configs... just a thought.

[th0ma7]

@ymartin59 forgot to mention, as the kernels are now built within the package directory tree (e.g. spksrc/spk/<package>/work*) this allows to play with the default kernel configuration without affecting the default kernel tree (spksrc/kernel/syno*). That should give much more flexibility to compile specific modules for various DSM kernels & versions.

[ymartin59]

@th0ma7 Ok, my interest is to achieve the idea of simplifying PR #3787 to deliver first a single package with all USB serial drivers, and eventually a custom do-it-yourself one driver package (for advanced users)