ARM: rmobile: Update little‐endian byte order option in srec_cat command

Since srecord v1.60, option "-Little_Endian_CONSTant" is deprecated.
Fix the build warnings by updating little‐endian byte order option in
srec_cat command when generating loader header.

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>