MFC r342882,r324979,r325115: sys/boot test scripts

[Note: arm/armv7 has been removed in the interest of not having a failing
architecture from the start]

MFC r342882: Create a shell script to build sys/boot on all the
architectures.

MFC r324979: Do a chdir to SRCTOP/sys/boot before starting the run.

MFC r325115: Fix parallel build issues and add MK_ZFS=no case