eNET: Remove config.mk

By including <config.h> in the ld script, CONFIG_SYS_MONITOR_LEN (defined
in the boards config file) can be used in lieu of FLASH_SIZE (defined in
the board specific config.mk)

As this is the last remaining entry in the board specific config.mk, this
file can now be removed

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>