build/nmk: declare build-as as a recursive

So, how it was working:
1. build-as was declared with $$(1) and $$(2) which were expanded
on entering the submake;
2. function $(call build-as,...) performed the second expansion of
build-as.

Cons: build-as works only in sub-makefile, no sub-sub-makefile, no
upper/top makefile.

Simplify this by single $(1).
Then build-as variable will be used _only_ in makefile, not in
sub-makefiles.

This is for now fine, as each file, that calls $(MAKE) with
$(build)=dir or $(call build-as,makefile,dir) will include main.mk
from NMK, which has build-as definition (from include.mk).

In the future, we'll get rid of $(build) and $(build-as) workarounds
as finally switch to building from a global makefile.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/nmk/scripts/include.mk

@@ -47,7 +47,7 @@ build := -r -R -f $(__nmk_dir)main.mk makefile=Makefile obj
 export build
 
 # With specified Makefile
-build-as := -r -R -f $(__nmk_dir)main.mk makefile=$$(1) obj=$$(2)
+build-as = -r -R -f $(__nmk_dir)main.mk makefile=$(1) obj=$(2)
 export build-as
 
 #