Combine block_erase*_en29lv640b and block_erase*_m29f400bt respectively.

This patch combines two identical block and chip erase functions respectively:
 - Merge block_erase_m29f400bt and block_erase_en29lv640b into
   erase_block_shifted_jedec.
 - Merge block_erase_chip_m29f400bt and block_erase_chip_en29lv640b into
   erase_chip_block_shifted_jedec.

Leave their implementations in en29lv640b.c for now.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

git-svn-id: svn://flashrom.org/flashrom/trunk@1808 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1