board: phytec: phycore_am64x: Move earlycon into own variable

By moving the earlycon definition into a dedicated variable, it's
easier to change these values in case the kernel should print on
a different serial interface.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>

board/phytec/phycore_am64x/phycore_am64x.env

@@ -11,7 +11,8 @@ mmcdev=1
 mmcroot=2
 mmcpart=1
 console=ttyS2,115200n8
-mmcargs=setenv bootargs console=${console} earlycon=ns16550a,mmio32,0x02800000
+earlycon=ns16550a,mmio32,0x02800000
+mmcargs=setenv bootargs console=${console} earlycon=${earlycon}
 	root=/dev/mmcblk${mmcdev}p${mmcroot} rootwait rw
 loadimage=load mmc ${mmcdev}:${mmcpart} ${loadaddr} Image
 loadfdt=load mmc ${mmcdev}:${mmcpart} ${fdtaddr} ${fdtfile}