Hi supriyaborchate,
Do you wish to create your own bootloader or just keep using an existing one from NXP?
FYI, the MBDT is delivered with a bootloader for S32K148 that can be found in the ..\NXP_MBDToolbox_S32K1xx\code\tools\BootLoader\RBF_Files as a RBF file.
In order to program this file on the target, all you need is the S32 Design Studio IDE and just flash the RBF file into the S32K148 Flash memory. Then you can use the RAppID Bootloader GUI to load applications via CAN or via UART interfaces.
In case you wish to develop you own bootloader, then you need to address your question here: S32K
Hope this helps!
Daniel