build: Implement CONFIGDIR setting

In effort to support different builds of Aimee OS using the same
scripts, without necessarily having to fork this repository, the build
system now supports a `CONFIGDIR` setting.  When this variable is set,
files defining the target environment, such as the lists of packages to
install, the kernel configuration, the Portage configuration, etc. are
found in the path it specifes.

The reference build, for the Home Assistant Yellow board, is configured
in the `yellow` directory.  To build it, run:

```sh
CONFIGDIR=yellow ./vm-build.sh
```

yellow/linux.config

@@ -0,0 +1,56 @@
+CONFIG_BTRFS_FS=y
+
+CONFIG_OVERLAY_FS=m
+
+CONFIG_MISC_FILESYSTEMS=y
+CONFIG_SQUASHFS=y
+CONFIG_SQUASHFS_XATTR=y
+CONFIG_SQUASHFS_ZSTD=y
+
+CONFIG_EFI=y
+CONFIG_EFI_STUB=y
+
+CONFIG_PCI=y
+CONFIG_PCIE_BRCMSTB=y
+CONFIG_BLK_DEV_NVME=y
+
+CONFIG_FW_LOADER_COMPRESS=y
+
+CONFIG_I2C=y
+CONFIG_I2C_CHARDEV=m
+CONFIG_I2C_MUX=m
+CONFIG_I2C_MUX_GPMUX=m
+CONFIG_I2C_MUX_PINCTRL=m
+CONFIG_I2C_BCM2708=m
+CONFIG_I2C_BCM2835=m
+CONFIG_I2C_BRCMSTB=m
+
+CONFIG_RTC_CLASS=y
+CONFIG_RTC_DRV_PCF85063=m
+
+CONFIG_LEDS_TRIGGERS=y
+CONFIG_LEDS_TRIGGER_DISK=y
+CONFIG_LEDS_TRIGGER_ACTIVITY=y
+
+CONFIG_WLAN=y
+CONFIG_WLAN_VENDOR_BROADCOM=y
+CONFIG_CFG80211=m
+CONFIG_MAC80211=m
+CONFIG_BRCM_FMAC=m
+CONFIG_BT=m
+
+# CONFIG_KSM is not set
+
+CONFIG_IPV6=y
+CONFIG_IPV6_SIT=m
+CONFIG_IPV6_SIT_6RD=m
+
+# CONFIG_MEDIA_CEC_SUPPORT is not set
+# CONFIG_MEDIA_SUPPORT is not set
+
+CONFIG_AUDIT=y
+CONFIG_SECURITY=y
+CONFIG_SECURITY_NETWORK=y
+CONFIG_SECURITY_SELINUX=y
+CONFIG_DEFAULT_SECURITY_SELINUX=y
+# DEFAULT_SECURITY_DAC is not set