lib/bitmap.c: enhance bitmap syntax

Today there are platforms with many CPUs (up to 4K).  Trying to boot only
part of the CPUs may result in too long string.

For example lets take NPS platform that is part of arch/arc.  This
platform have SMP system with 256 cores each with 16 HW threads (SMT
machine) where HW thread appears as CPU to the kernel.  In this example
there is total of 4K CPUs.  When one tries to boot only part of the HW
threads from each core the string representing the map may be long...  For
example if for sake of performance we decided to boot only first half of
HW threads of each core the map will look like:
0-7,16-23,32-39,...,4080-4087

This patch introduce new syntax to accommodate with such use case.  I
added an optional postfix to a range of CPUs which will choose according
to given modulo the desired range of reminders i.e.:

    <cpus range>:sed_size/group_size

For example, above map can be described in new syntax like this:
0-4095:8/16

Note that this patch is backward compatibl...