Makefile: use EXTRACFLAGS in order to use user's flags

The main reason to introduce this EXTRACFLAGS variable is to allow people
to cross compile on x86 architectures over 32bits and 64bits

    make EXTRACFLAGS="-m32"
    make EXTRACFLAGS="-m64"

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Acked-by: Alessandro Rubini <rubini@gnudd.com>

tools/Makefile

 
-CFLAGS = -ggdb -Wall -I../kernel
+CFLAGS = -ggdb -Wall -I../kernel $(EXTRACFLAGS)
 LDFLAGS = -L. -lspec
 
 LIB = libspec.a