os/tools: modify to change filename to 'tizenrt'
os/tools: modify to change filename to 'tizenrt'

When run to "make export" command, The script file 'mkexport.sh' gathers
each object file(.o) and creates as compressed file with the prefixed
"nuttx". This commit modifies the "nuttx" to "tizenrt" in filename.

Change-Id: I797b30f7db764cf1cd21a0a7d023ffd41f44f954
Signed-off-by: Jihun Ahn <31fe625dd60d1475d1b18ea57157c450e67c870a@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT