fs: shell: Add more shell functions
fs: shell: Add more shell functions

Add following commands to fs shell:
- rf <filename>
- read <filename> [<offset> [<length>]]
- write <filename> [-o <offset>] <byte1> [<byte2> ... [<byteN>]]
- trunc <filename> [<length>]

For writes without offset specified, new data is appended to file.
For truncaces without length specified, 0 is default value.

Signed-off-by: Andrzej Kaczmarek <e629a10adbe722145f074acb1ca27e72d94cfcb5@codecoup.pl>

aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyriot/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,kraj/zephyr,galak/zephyr,galak/zephyr,mbolivar/zephyr,ldts/zephyr,mbolivar/zephyr,punitvara/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,mbolivar/zephyr,finikorg/zephyr,mbolivar/zephyr,punitvara/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,punitvara/zephyr,zephyriot/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,explora26/zephyr,punitvara/zephyr,zephyriot/zephyr,ldts/zephyr