Create uboot.md

docs/uboot.md

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+#### build
+
+```
+git clone https://github.com/u-boot/u-boot
+make distclean
+make rpi_4_defconfig
+make -j X
+```
+
+#### config.txt
+
+```
+kernel=u-boot.bin
+kernel_address=Ox00080000
+arm_64bit=1
+enable_uart=1
+uart_2ndstage=1
+enable_gic=1
+```