sgi: Fix ext3 partition size in virtual disk partition table

The size of ext3 partition in the partition table was incorrectly set to
$((ext3_part_size-1)). The size should match the size of the allocated
parittion while creating the partition. So fix this mismatch in multiple
build scripts, viz. build-test-busybox.sh, build-test-buildroot.sh,
build-test-secureboot.sh.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Ie5b4b42dcb98b04e66c941bcbaaf03e2d5b5f10d