1. change jffs2 size calc method. it should be a bug while set it's size. it was set as size= PART_JFFS2_MIN * mtd->erasesize
2. add a PART_JFFS2_GAP build CFLAG to target.mak. PART_JFFS2_GAP was used in linux/linux-2.6/arch/mips/brcm-boards/bcm947xx/nonand-setup.c
and defined as 5 blocks (256k). lots of current router has flash more than 16Mb. the 256k limit is just too small.
by adding this build option. we can set it's value in Makefile. or just set "0" to use all unused space.
3. some mksquashfs use squashfs-lzma instead of squashfs for rootfs. we correct some detect method in router/rc and router/httpd
Change-Id: I8a5897b2e26ddea8b9b633f49fa9b336070b4fab