nrf: Improve include of boardmodules.mk
nrf: Improve include of boardmodules.mk

Removing shell commands for checking if boardmodules.mk exists
under boards/<BOARD>/modules folder before including it.

This patch does the equivalent to previous test without using
shell commands. Hence, including the .mk if it exists.

Reference:
https://stackoverflow.com/questions/8346118/check-if-a-makefile-exists-before-including-it

tralamazza/micropython,tralamazza/micropython,tralamazza/micropython,tralamazza/micropython