boot: Fix ifdefs in DIRECT_XIP and RAM_LOAD case
boot: Fix ifdefs in DIRECT_XIP and RAM_LOAD case

The function `boot_erase_region` is used in some cases when DIRECT_XIP
or RAM_LOAD are defined, however it is specifically not compiled in in
these cases.  Correct the ifdefs so this function will be available.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot