configs/ts7680: bump Linux to 4.9, to fix build with gcc >= 8.x

configs/ts7680: bump Linux to 4.9, to fix build with gcc >= 8.x

The TS-7680 defconfig does not build with gcc 8.x and 9.x because it
uses an old 3.14 kernel. Technologic Systems, the board manufacturer
recently released an updated 4.9 based kernel on a separate repository
on github.

Bump the kernel release from 3.14.28 to 4.19.186 and update the linux
defconfig name as requested in the TS-7680 documentation [1].

[1] https://wiki.embeddedarm.com/wiki/TS-7680#Linux_4.9.y

Signed-off-by: Julien Grossholtz <julien.grossholtz@openest.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>