[build] Properly handle multiple goals per BIN directory

When building multiple targets per BIN with multiple jobs, for
example:

  make -j16 bin-i386-efi/ipxe.efi{,drv,rom} bin-x86_64-efi/ipxe.efi{,drv,rom}

we would invoke a make subprocess for each goal in parallel resulting
in multiple makes running in a single BIN directory.  Fix by grouping
goals per BIN directory and invoking only one make per BIN.  It is
both safer and faster.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

@@ -21,6 +21,7 @@ CP		:= cp
 ECHO		:= echo
 PRINTF		:= printf
 PERL		:= perl
+TRUE		:= true
 CC		:= $(CROSS_COMPILE)gcc
 CPP		:= $(CC) -E
 AS		:= $(CROSS_COMPILE)as