Fix high memory zone initialization in CSM mode

Fix high memory zone initialization in CSM mode

malloc_high() cannot allocate any memory in CSM mode due to an empty
ZoneHigh. SeaBIOS cannot find any disk to boot from because device
initialization fails.

The bug was introduced in 1.16.1 (commit dc88f9b) when the meaning of
BUILD_MAX_HIGHTABLE changed but CSM code was not updated. This patch
reverts to the previous behavior by using BUILD_MIN_HIGHTABLE in CSM
methods.

Signed-off-by: José Martínez <xose@google.com>