package/uboot-tools: fix MKIMAGE_ARCH handling

package/uboot-tools: fix MKIMAGE_ARCH handling

Fix MKIMAGE_ARCH handling to avoid the following build failure:

/home/buildroot/autobuild/instance-0/output-1/build/host-uboot-tools-2021.07/tools/mkimage -C none -A openrisc -T script -d /home/buildroot/autobuild/instance-0/output-1/boot_script.txt /home/buildroot/autobuild/instance-0/output-1/build/host-uboot-tools-2021.07/tools/boot.scr

Invalid architecture, supported are:
	alpha            Alpha
	arc              ARC
	arm              ARM
	arm64            AArch64
	avr32            AVR32
	blackfin         Blackfin
	ia64             IA64
	invalid          Invalid ARCH
	m68k             M68K
	microblaze       MicroBlaze
	mips             MIPS
	mips64           MIPS 64 Bit
	nds32            NDS32
	nios2            NIOS II
	or1k             OpenRISC 1000
	powerpc          PowerPC
	riscv            RISC-V
	s390             IBM S390
	sandbox          Sandbox
	sh               SuperH
	sparc            SPARC
	sparc64          SPARC 64 Bit
	x86              Intel x86
	x86_64           AMD x86_64
	xtensa           Xtensa

Strangely enough, we only have autobuilder failures since July 2021 even
as or1k has been used since the addition of openriscv support in 2012:
https://github.com/u-boot/u-boot/commit/3ddcaccda3824e1c7f7266d543e4c0eb3ea9851c

For x86)64, we incorrectly mangle it to x86.

Finally, the comment about mips64 is wrong: mips64 *is* a valid
archtecture, and we anyway had no code to tweak that case.

Fixes:
 - http://autobuild.buildroot.org/results/c3f0f2a3fb87d74bfdaccf9b94c66f0b5bae7520

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: extend commit log for mips64 and x86_64]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>