mynewt: Allow custom boot to be provided by pkg

BOOT_CUSTOM_START is defined by APP package (here).
This changes default value from 0 to empty
to allow for constructing packege that provides function
boot_custom_start() and automatically sets BOOT_CUSTOM_START to 1
for easy setup.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>