Disable concurrent building entirely

Two reasons for doing this:

* The current solution for enforcing sequential operations makes
  a lot of things slower. Either we do it properly or we don't.

* Most of Erlang.mk is sequential by nature. There is very little
  to gain from Make's own parallel building. Something more
  interesting would be to enable fetching of dependencies
  concurrently by other means, and enable compilation of
  Erlang files concurrently (requires patching OTP).

core/core.mk

@@ -73,13 +73,9 @@ endif
 
 # Core targets.
 
-ifneq ($(words $(MAKECMDGOALS)),1)
 .NOTPARALLEL:
-endif
 
-all:: deps
-	$(verbose) $(MAKE) --no-print-directory app
-	$(verbose) $(MAKE) --no-print-directory rel
+all:: deps app rel
 
 # Noop to avoid a Make warning when there's nothing to do.
 rel::
@@ -101,7 +97,7 @@ help::
 		"erlang.mk (version $(ERLANG_MK_VERSION)) is distributed under the terms of the ISC License." \
 		"Copyright (c) 2013-2015 Loïc Hoguin <essen@ninenines.eu>" \
 		"" \
-		"Usage: [V=1] $(MAKE) [-jNUM] [target]..." \
+		"Usage: [V=1] $(MAKE) [target]..." \
 		"" \
 		"Core targets:" \
 		"  all           Run deps, app and rel targets in that order" \

doc/src/guide/getting_started.asciidoc

@@ -228,7 +228,7 @@ $ make help
 erlang.mk (version 1.2.0-642-gccd2b9f) is distributed under the terms of the ISC License.
 Copyright (c) 2013-2015 Loïc Hoguin <essen@ninenines.eu>
 
-Usage: [V=1] make [-jNUM] [target]...
+Usage: [V=1] make [target]...
 
 Core targets:
   all           Run deps, app and rel targets in that order