kbuild: Bump the build system to 5.1

Our last sync with the kernel was 5.1. Even that was a partial one
as some patches from 4.x kernels were already missing making the
transition to a modern kbuild infeasible.

We are so out of sync now, that tracking the patches and backporting
them one by one makes little sense and it's going to take ages.

This is an attempt to sync up Makefile[.lib/.kbuild].
Unfortunately due to sheer amount of patches this is not easy to review,
but that's what we decided during a community call.

One of the biggest changes is get rid of partial linking entirely and
build .a archives isntead of .o.
We diaviate from the kernel on that. Instead of calling a custom script
to create the archive symbol table, we call ar with rcTP (isntead of
rcSTP) since we want a resulting archive that's sauble with the linker.

The only affected platforms are PPC ones. Unfortunately I don't have any
of them around to test, but the objdump of the resulting files --
arch/powerpc/lib/built-in.[oa] looks identical.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/develop/crash_dumps.rst

@@ -89,15 +89,15 @@ File u-boot.map contains the memory layout of the U-Boot binary. Here we find
 these lines::
 
    .text.do_undefined
-                  0x00000000000101fc        0xc cmd/built-in.o
+                  0x00000000000101fc        0xc cmd/built-in.a
    .text.exception_complete
-                  0x0000000000010208       0x90 cmd/built-in.o
+                  0x0000000000010208       0x90 cmd/built-in.a
    ...
    .text.cmd_process
-                  0x00000000000213b8      0x164 common/built-in.o
+                  0x00000000000213b8      0x164 common/built-in.a
                   0x00000000000213b8                cmd_process
    .text.cmd_process_error
-                  0x000000000002151c       0x40 common/built-in.o
+                  0x000000000002151c       0x40 common/built-in.a
                   0x000000000002151c                cmd_process_error
 
 So the error occurred at the start of function do\_undefined() and this