tools: fix env_flags build

$ make tools-all
  ...
  In file included from tools/env/env_flags.c:1:0:
  tools/env/../../common/env_flags.c: In function
    ‘env_flags_parse_varaccess_from_binflags’:
    tools/env/../../common/env_flags.c:156:18: warning: implicit declaration
    of function ‘ARRAY_SIZE’ [-Wimplicit-function-declaration]
      for (i = 0; i < ARRAY_SIZE(env_flags_varaccess_mask); i++)
                      ^
Seems like the other utilities just add a copy of ARRAY_SIZE since
there's nowhere to include it from (tools/imagetool.h,
tools/mxsimage.h). Let's do the same here.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>