add the interrupt_bootstrap directed test to verify mtvec_addr_i pins

Created by: strichmo

Very simple directed test of mtvec_addr_i.

Also adds following:

• Adds plusarg hook to set mtvec_addr_i as a short-term fix to set pins using existing interface initial block
• Add common makefile target to allow tests with both assembly and C source. The bootstrap test uses this to implement a different start routine to avoid BSP's setting of mtvec.

Signed-off-by: Steve Richmond Steve.Richmond@silabs.com