[Thinkpad Tablet Android Development] Phoenix09:
interestingly, I get a different error:

CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
CC arch/arm64/mm/dma-mapping.o
In file included from ./arch/arm64/include/asm/dma-iommu.h:12:0,
from arch/arm64/mm/dma-mapping.c:33:
include/linux/dma-mapping-fast.h:46:15: warning: 'struct dma_iommu_mapping' declared inside parameter list
error, forbidden warning: dma-mapping-fast.h:46
scripts/Makefile.build:257: recipe for target 'arch/arm64/mm/dma-mapping.o' failed
make[1]: *** [arch/arm64/mm/dma-mapping.o] Error 1
Makefile:946: recipe for target 'arch/arm64/mm' failed
make: *** [arch/arm64/mm] Error 2

and using the kernel config extracted from the kernel in build 180906 for the TB-8704X (I don't have the tablet yet):

CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
CHK kernel/config_data.h
CC drivers/net/ethernet/msm/rndis_ipa.o
In file included from drivers/net/ethernet/msm/rndis_ipa_trace.h:81:0,
from drivers/net/ethernet/msm/rndis_ipa.c:32:
include/trace/define_trace.h:83:43: fatal error: ./rndis_ipa_trace.h: No such file or directory
#include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
^
compilation terminated.
scripts/Makefile.build:257: recipe for target 'drivers/net/ethernet/msm/rndis_ipa.o' failed
make[4]: *** [drivers/net/ethernet/msm/rndis_ipa.o] Error 1
scripts/Makefile.build:402: recipe for target 'drivers/net/ethernet/msm' failed
make[3]: *** [drivers/net/ethernet/msm] Error 2
scripts/Makefile.build:402: recipe for target 'drivers/net/ethernet' failed
make[2]: *** [drivers/net/ethernet] Error 2
scripts/Makefile.build:402: recipe for target 'drivers/net' failed
make[1]: *** [drivers/net] Error 2
Makefile:946: recipe for target 'drivers' failed
make: *** [drivers] Error 2

I'll play with it some more when I get mine sometime next week and update it and extract the kernel config

I haven't had any luck playing with merging the TB-X704L and TB-8704X sources, still getting the same error.

Edit: I get the same error for the TB-X704L and TB-X704F.

Another edit: did a diff with the Pixel kernel:

diff -ur tab48plus/drivers/net/ethernet/msm/rndis_ipa_trace.h msm/drivers/net/ethernet/msm/rndis_ipa_trace.h
--- tab48plus/drivers/net/ethernet/msm/rndis_ipa_trace.h 2017-05-02 05:06:34.000000000 +0100
+++ msm/drivers/net/ethernet/msm/rndis_ipa_trace.h 2019-04-08 13:21:43.806756700 +0100
@@ -77,5 +77,5 @@

/* This part must be outside protection */
#undef TRACE_INCLUDE_PATH
-#define TRACE_INCLUDE_PATH .
+#define TRACE_INCLUDE_PATH ../../drivers/net/ethernet/msm
#include <trace/define_trace.h>

now I've gotten further, there's a lot of these errors

After fixing all the incorrect includes and adding the missing files, I have a kernel image

Edit: https://github.com/Phoenix09/android_kernel_lenovo_tb8704
[MENTION=9701462]code_exec[/MENTION]: you should probably extract the config from your device and try building with this repo, I have no idea if any of the configs in the source are for this tablet.