arc: Update variables substitutions in u-boot env files

In the latest version of u-boot (2018.05) there was a swith to
Hush shell for ARC AXS10x boards(arc770/archs38):
commit 9249d74781e1 ("ARC: AXS10x: Enable hush shell").
In Hush shell using "$()" to declare envitonment variables is forbidden,
instead of this "${}" need to be used.

Signed-off-by: Evgeniy Didin <[email protected]>
Cc: Alexey Brodkin <[email protected]>
Cc: Hauke Mehrtens <[email protected]>
Cc: John Crispin <[email protected]>

target/linux/arc770/image/uEnv.txt

@@ -1,7 +1,7 @@
 setenv kernel_addr_r 0x82000000
 setenv fdt_addr_r 0x83000000
-setenv loadkernel fatload mmc 0 \$(kernel_addr_r) uImage
-setenv loaddtb fatload mmc 0 \$(fdt_addr_r) \$(dts)
+setenv loadkernel fatload mmc 0 \${kernel_addr_r} uImage
+setenv loaddtb fatload mmc 0 \${fdt_addr_r} \${dts}
 setenv bootargs earlycon=uart8250,mmio32,0xe0022000,115200n8 console=ttyS3,115200n8 root=/dev/mmcblk0p2 rootwait print-fatal-signals=1
-setenv uenvcmd run loadkernel\; run loaddtb\; bootm \$(kernel_addr_r) - \$(fdt_addr_r)
+setenv uenvcmd run loadkernel\; run loaddtb\; bootm \${kernel_addr_r} - \${fdt_addr_r}
 run uenvcmd

target/linux/archs38/image/uEnv.txt

@@ -1,7 +1,7 @@
 setenv kernel_addr_r 0x82000000
 setenv fdt_addr_r 0x83000000
-setenv loadkernel fatload mmc 0 \$(kernel_addr_r) uImage
-setenv loaddtb fatload mmc 0 \$(fdt_addr_r) \$(dts)
+setenv loadkernel fatload mmc 0 \${kernel_addr_r} uImage
+setenv loaddtb fatload mmc 0 \${fdt_addr_r} \${dts}
 setenv bootargs earlycon=uart8250,mmio32,0xe0022000,115200n8 console=ttyS3,115200n8 root=/dev/mmcblk0p2 rootwait print-fatal-signals=1
-setenv uenvcmd run loadkernel\; run loaddtb\; bootm \$(kernel_addr_r) - \$(fdt_addr_r)
+setenv uenvcmd run loadkernel\; run loaddtb\; bootm \${kernel_addr_r} - \${fdt_addr_r}
 run uenvcmd