Remove 'boot2' target from the MIPS version of boot2: instead define
'jtagboot' and 'flashboot' intended to be used via JTAG load into DRAM
and written to flash, respectively.

Break out relocate.S from start.S: the former is the self-relocation
code, which is not needed for JTAG boot, but is needed for flash boot.

Break out linker.cfg into different linker scripts: common.ldscript,
shared by both configurations, and then jtag/flash-specific linker
scripts.