defconfig: add CONFIG_SERIAL_RX_BUFFER to avoid drop from bootmenu

uenv: r4: drop memory limit in bootargs/cmdline

this was a leftover from early development state where switch-,
ethernet- and ppe driver did not support 4G ram to get them to work.

fix: readded missing lines to mt7986a-bpi-r3-emmc.dts to

address the following error when cold-booting from the emmc.

NOTICE:  BL2: v2.6(release):c30a1caf827-dirty emmc
NOTICE:  BL2: Built : 20:12:40, Feb  7 2024
NOTICE:  WDT: disabled
NOTICE:  CPU: MT7986 (2000MHz)
NOTICE:  EMI: Using DDR4 settings
NOTICE:  EMI: Detected DRAM size: 2048MB
NOTICE:  EMI: complex R/W mem test passed
ERROR:   MSDC: Command has timed out with cmd=8, arg=0x1aa
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   BL2: Failed to load image id 3 (-5)

fix: readded missing lines to mt7986a-bpi-r3-emmc.dts to

address the following error when cold-booting from the emmc.

NOTICE:  BL2: v2.6(release):c30a1caf827-dirty emmc
NOTICE:  BL2: Built : 20:12:40, Feb  7 2024
NOTICE:  WDT: disabled
NOTICE:  CPU: MT7986 (2000MHz)
NOTICE:  EMI: Using DDR4 settings
NOTICE:  EMI: Detected DRAM size: 2048MB
NOTICE:  EMI: complex R/W mem test passed
ERROR:   MSDC: Command has timed out with cmd=8, arg=0x1aa
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   BL2: Failed to load image id 3 (-5)

fix: readded missing lines to mt7986a-bpi-r3-emmc.dts to

address the following error when cold-booting from the emmc.

NOTICE:  BL2: v2.6(release):c30a1caf827-dirty emmc
NOTICE:  BL2: Built : 20:12:40, Feb  7 2024
NOTICE:  WDT: disabled
NOTICE:  CPU: MT7986 (2000MHz)
NOTICE:  EMI: Using DDR4 settings
NOTICE:  EMI: Detected DRAM size: 2048MB
NOTICE:  EMI: complex R/W mem test passed
ERROR:   MSDC: Command has timed out with cmd=8, arg=0x1aa
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   MSDC: Command has timed out with cmd=17, arg=0x680000
ERROR:   BL2: Failed to load image id 3 (-5)

Merge pull request #15 from meehien/2024-01-bpi-emmc

fix: readded missing lines to mt7986a-bpi-r3-emmc.dts to

readme: update usage steps for mtk arm64

to include ATF steps

uenv: r3/r4: update builtin environment

env: r3: add bootnvme without asking but loading env from nvme_boot

ci: try to build bl2 with ubi support for R4