wic: Make disk partition size consistently computed

wic: Make disk partition size consistently computed

When using different root directories with a wks file wic is using the
value from the original ROOTFS_SIZE which is not correct.  Example:

Number  Start   End     Size    File system  Name       Flags
 1      20.5kB  318MB   318MB   fat16        otaefi     legacy_boot, msftdata
 2      318MB   636MB   318MB   ext4         otaboot
 3      636MB   1709MB  1074MB  ext4         otaroot
 4      1709MB  2027MB  318MB   ext4         otaboot_b
 5      2027MB  3101MB  1074MB  ext4         otaroot_b
 6      3101MB  5249MB  2147MB  ext4         fluxdata

The partitions 1, 2, and 3 incorrectly inherit the size, instead of using
a computed size.  With the patch applied it is working properly:

Number  Start   End     Size    File system  Name       Flags
 1      20.5kB  14.5MB  14.5MB  fat16        otaefi     legacy_boot, msftdata
 2      14.5MB  65.3MB  50.8MB  ext4         otaboot
 3      65.3MB  1139MB  1074MB  ext4         otaroot
 4      1139MB  1190MB  50.8MB  ext4         otaboot_b
 5      1190MB  2264MB  1074MB  ext4         otaroot_b
 6      2264MB  4411MB  2147MB  ext4         fluxdata

As for the removal of the bb.warn, if the size is not specified, it is
not something to warn the end user about.  Some of my default images
make use of the head room + a computed directory size or while generating
images.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>