Cosmetic: Fix a number of typos, no functional changes.

Fix various misspellings of things like "environment", "kernel",
"default" and "volatile", and throw in a couple grammar fixes.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
diff --git a/doc/README.hwconfig b/doc/README.hwconfig
index cf54965..b6ddb43 100644
--- a/doc/README.hwconfig
+++ b/doc/README.hwconfig
@@ -18,7 +18,7 @@
    U-Boot. But I haven't bothered yet.
 
 2. Since we don't implement a hwconfig command, i.e. we're working
-   with the environement directly, there is no way to tell that
+   with the environment directly, there is no way to tell that
    toggling a particular option will need a reboot to take
    effect. So, for now it's advised to always reboot the
    target after modifying the hwconfig variable.