u-boot: do not clobber PARALLEL_MAKE setting

u-boot: do not clobber PARALLEL_MAKE setting

The u-boot tree is fully capable of parallel builds, so this
setting should not exist as a blanket setting for all of the
recipes.  Going forward, if there is a parallelism issue
in u-boot, it needs to be reported and fixed there, and not
with the "make -j1" band-aid approach.

(From OE-Core rev: 7fdd1f82803df9752e908f01f7643d66b82a690e)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>