How can I start different programs from flash.

Your objcopy solution to relocation is a great idea; a really innovative solution to multi-boot. It will continue to work just fine in the future because the boot-loader is fully relocateable.

As for parameterizing your 'asm("call 0x200000");' statement, take a look in the GNU documentation included with the kit under "Assembler Instructions with C Expression Operands". Just about any permutation of "take the value of a C variable and manipulate it in assembly" is covered there.