armv7: start.S: fixes and enhancements for SPL

- Allow SPL to have .bss disjoint from rest of the image
- Allow for .bss setup in CONFIG_SPL_BUILD case too.
- Take care of the special case where relocation offset = 0.
- Compile out exception handling code and install a simpler
  vector

Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>