samples/shell: print the actual params passed

The params command is not printing the arguments, which makes this not
so useful to verify how things work, so added it to the sample.

Change-Id: I2d71ee064f12c7353ba83144824ae46803e2ad39
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>