Convert CONFIG_SYS_MEMTEST_START et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MEMTEST_START
   CONFIG_SYS_MEMTEST_END

This is removing unused defines and correcting the default value to be
0x0 as we are a hex symbol.

Signed-off-by: Tom Rini <trini@konsulko.com>