image: fix Linksys image alignment and simplify footer creation

image: fix Linksys image alignment and simplify footer creation

Current factory image sizes for Linksys devices are 256-byte aligned. This
is not an issue writing factory images from the OpenWrt or Linksys GUIs,
but can lead to failures using a TFTP client from the Linksys bootloader:

     NAND write: device 1 offset 0x2800000, size 0xc00100
     Attempt to write to non page aligned data
     NAND write to offset 2800000 failed -22
      0 bytes written: ERROR

Simplify Linksys footer creation by migrating to a makefile build recipe,
and pre-pad the footer (with 0xFF) to ensure the final image is $(PAGESIZE)
aligned.  Finally, remove the old linksys-image.sh script no longer needed.

Linksys footer details are given below for future reference. The 256-byte
footer is appended to factory images and tested by both the Linksys
Upgrader (observed in EA6350v3) and OpenWrt sysupgrade.

  Footer format:
    .LINKSYS.     Checked by Linksys upgrader before continuing.  (9 bytes)
    <VERSION>     Upgrade version number, unchecked so arbitrary. (8 bytes)
    <TYPE>        Model of device, space padded (0x20).          (15 bytes)
    <CRC>         CRC checksum of factory image to flash.         (8 bytes)
    <padding>     Padding ('0' + 0x20 * 7)                        (8 bytes)
    <signature>   Signature of signer, unchecked so arbitrary.   (16 bytes)
    <padding>     Padding with nulls (0x00)                     (192 bytes)

Link: https://github.com/openwrt/openwrt/pull/11405#issuecomment-1358510123
Link: https://github.com/openwrt/openwrt/pull/11405#issuecomment-1587517739
Reported-by: Stijn Segers <foss@volatilesystems.org>
Reported-by: Wyatt Martin <wawowl@gmail.com>
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>