kernel since 5.6 do not boot anymore on Apple PowerBook

[Giuseppe Sacco]

Hello,
while trying to debug a problem using git bisect, I am now at a point
where I cannot build the kernel at all. This is the error message I
get:

$ LANG=C make ARCH=powerpc \
 CROSS_COMPILE=powerpc-linux- \
 CONFIG_MODULE_COMPRESS_GZIP=true \
 INSTALL_MOD_STRIP=1 CONFIG_MODULE_COMPRESS=1 \
 -j4 INSTALL_MOD_PATH=$BOOT INSTALL_PATH=$BOOT \
 CONFIG_DEBUG_INFO_COMPRESSED=1 \
 install modules_install
make[2]: *** No rule to make target 'vmlinux', needed by
'arch/powerpc/boot/zImage.pmac'.  Stop.
make[1]: *** [arch/powerpc/Makefile:407: install] Error 2
make: *** [Makefile:328: __build_one_by_one] Error 2

How can I continue?

Thank you,
Giuseppe