imgtool custom TLV disregards endian and always uses big endian

[nordicjm]

This was originally (a long time ago) reported to the wrong repository: apache/mynewt-mcumgr#152

The issue with imgtool is that the value of --custom-tlv is always encoded in big-endian format, one might assume that selecting the endian of the output would determine this but it does not, and according to https://github.com/mcu-tools/mcuboot/blob/main/boot/bootutil/include/bootutil/boot_status.h#L156 actually should not because all fields must be in little endian, therefore imgtool has a bug in this functionality.

Example:

imgtool sign -k /tmp/aa/bootloader/mcuboot/root-rsa-2048.pem --align 4 --version 1.2.3 -H 0x200 -S 0x78000 --custom-tlv 0xaa 0x22550011 hello_world/zephyr/zephyr.hex test.hex

The following is the raw bytes from the hex file (taken from JFlash): 00 AA 00 04 00 22 55 00 11, as can be seen 0x22550011 is wrongly added in big endian format.

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