uboot-sign.bbclass: Refactor condition checks to use && and || instead of -a and -o

uboot-sign.bbclass: Refactor condition checks to use && and || instead of -a and -o

This commit cleans up and modernizes the shell condition expressions in
`uboot-sign.bbclass` to follow best practices for portable and reliable shell usage.

Key changes:
- Replace legacy `[ -a ]` and `[ -o ]` with explicit `[ ] && [ ]` and `[ ] || [ ]`.
  Modern POSIX and busybox sh recommend using `&&` and `||` instead of `-a` and `-o`
  because `-a` and `-o` are less robust and can cause parsing ambiguities in some shells.
- Simplify `concat_dtb()` by moving the DTB existence check to the top and using
  early `return` to avoid deep nesting.
- Remove redundant fallback `else` blocks; use clearer control flow with direct checks.

This improves maintainability, reduces shell syntax pitfalls, and aligns with
current shell scripting best practices.

References:
- POSIX recommends avoiding `-a` and `-o` in `[ ]` and using explicit `&&` and `||`:
  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>