manflash: load .payloadvectors from elf too.
Fix interrupt enable durung application erase.

Interrupts were enabled too early during application erase, before the modified
interrupt table could be written. If an interrupt occoured at this point, the
bootloader would reset and the uploader could not write the first application
page.

This has been fixed by only enabling interrupts when they were enabled before
calling cli() in writeFlashPage() and writeWordToPageBuffer().

Interestingly, program size got smaller by 4 bytes too.