alpine: disable parallel build

alpine: disable parallel build

Parallel build does not work because it may reach a point where OSTYPE
might be needed before it is actually built.  They appear to run
parallel to each other:

echo slx > OSTYPE
[...]
cat: OSTYPE: No such file or directory
sh: line 1: test: too many arguments
cat: OSTYPE: No such file or directory
Already built for -- you must do "make clean" first
make[6]: *** [Makefile:706: rebuild] Error 1
make[6]: *** Waiting for unfinished jobs....

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>