linux/const.h: import from kernel

Combine the uapi/linux/const.h header into the kernel linux/const.h. The
next commit will use the _AC macro this header instead of the common.h
definition.

Based on Linux kernel version 4.19.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Stefan Roese <sr@denx.de>