Devmem2

Add support for the devmem2 tool in android system.It can read and write physical addresses.

1. How to create the "/dev/mem" node in android system
   The "/dev/mem" node is not found in the default andorid system.Please enable the CONFIG_DEVMEM option in android linux kernel.The CONFIG_DEVMEM option is used to control whether to create the "/dev/mem" node. More details can be found in "kernel/drivers/char/mem.c".

NOTE1: The DEVKMEM option is not supported on arm64. On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write.
NOTE2: [Operation not permitted], if you get this error, please disable CONFIG_STRICT_DEVMEM
and CONFIG_IO_STRICT_DEVMEM option.

2. How to compile
   Enter the devmem2 source directory:
   $ cd ${MY_ANDROID}/external
   $ git clone https://github.com/giraffesnn/devmem2.git
   $ source build/envsetup.sh
   $ lunch # Choose the platform/board
   $ mmm external/devmem2
   $ make snod # make system.img ignoring depencies